Ok - just got home from work so i figured i'd give you a brief type up:
(i'm going to stick with freshwater as a saltwater setup would require a lot of maintenance and as a beginner a salt water setup may frustrate you out of the hobby)
1.
Fish:
There's a handful of ways that you can go here, so I'll break this down into three categories - community fish, aggressive fish and bottom feeders. Community fish tend to be the non aggressive types - you can mix a variety of them together without issues and can typically create schools of said fish. Some examples will be tetras, livebearers (literally means they give live birth - and include swordtails, platies and mollies), danios, Angel Fish (to a certain degree), etc. These fish will be easy to care for and will generally get along. However, you will make a few sacrifices if you decide to go this route - these fish tend to have less personality and tend to be less colorful. Aggressive fish can be exciting to keep but you need to approach this with caution. Under no circumstances should they be mixed with the prior mentioned category - they will not get along and you will end up with a lot of dead fish. Examples of some aggressive fish are cichlids (generally separated into two categories "New World" and "Old World" - which I will get into a little later). Now these fish tend to be much more colorful and exciting to keep but you will need to provide them in something resembling their natural habitat. Where i'm going with this is - is there is a concept with these fish called "spreading out the aggression". Meaning they will need lots of hiding places and a good amount of fish in the tank. This will prevent any aggression being singled out on any one fish and provide plenty of hiding spaces for anyone being picked on to escape. The third category is bottom feeders. I categorized them on their own since although they are community / peaceful fish they can often be mixed in with either aggressive or community fish and will provide a lot of utility towards your tank. Fish in this category will include Plecos (algae eaters), catfish (cory's, synodontis - think small), loaches, etc. Good thing about adding these guys in is they will contribute towards the tank maintenance - Pleco's will eat algae off the glass, catfish will eat leftover food / waste off the substrate, etc.
2.
Tank Size:
Think of what you were ideally going with and add 10 gallons to it. Small tanks are appealing because of the size (especially in an apartment) but they have their downside. You are limited in the types of fish you can have, typically smaller tanks get dirtier faster and are more difficult to clean, if there is a water issue it will spread faster and be more difficult to correct, etc. The usually rule of thumb is you can get one inch of fish per 1 gallon of water - although there are many out there who argue that it should rather be a function of surface area.. so the larger area you have in the tank the more fish you can add. I can't remember it off the top of my head but it's definite worth a google search. Without knowing any of your restrictions I would encourage you to look at something 55 gallons or larger - if you wanted to go with community fish you could go as low as 29 gallons. (I realize these sound like odd numbers but they are the standard sizes in which tanks come in, so if you buy in one of these standardized sizes 10, 20, 29, 40 gallon breeder, 55, 75, 90, 110, 125). I wouldn't be surprised if you could get a 55 with a stand and the whole 9 for ~$100.
3.
Filtration, Substrate, Heaters, etc.
Filtration - most important part of the tank. You can go two routes, first is a filter that hangs off the back of the tank (I would recommend the aqua clear 110) or a canister filter (which typically sits under the tank - normally used for larger tanks). This really depends on your budget, the size of the tank and the type of fish you plan on keeping. So if you want to get "dirtier fish" (think Cichlids, catfish, etc.) you may want to ramp up the filtration and if you have fish that don't create as much waste you can do something a bit smaller. There are also two different types of filtration you can go with in that substrate. The first is Mechanical, which will utilize activated carbon to filter the water. The second is biological, where you would utilize different layers of filtration (I've seen people use ceramics, sponges, etc.) that does not use chemicals to filter. The reason i recommended the aqua clear is you can choose either / or - as oppose to some other filters that hang on the back that only take remade filter cartridges. If you decide to go with a canister they can be quite expensive - I would recommend the sunsun 304B which I have heard great things about... and you can get it on ebay or amazon for less then $80. The size of your tank / type of fish will dictate how big of a filter you need - but the general rule of thumb would be 3-4 times the size of the tank in gallons per hour of outflow. I'm going to install two SunSun304B's in my 110 which will pump out between 800-1000 gallons of water an hour.
Substrate - again depends on which kind of fish you want to get. Some substrates will boost the PH in the water and create water conditions hospitable to your fish. If you wanted to do live plants you would need to get a special type of substrate that includes nutrients. You could do sand - which is a bit easier to clean (especially with a canister filter). Really whatever is most appealing to you. I would just advise against getting the gimmicky substrates (pink, glow in the dark, or any other unnatural color) - it may look neat at first but it will make the tank look unnatural and tacky.
Heaters - if you are going to get anything outside of goldfish you will need a simple heater. Typically you want the temperature of the tank to be between 68-76 degrees depending on the species of fish. You can monitor this with a tank thermometer - I recommend getting one a digital read out. If you get a bigger tank I would get two for each side of the tank - if you go with one just make sure it NOT right next to the heater or you will get inaccurate readings. (something like this would be ideal
)
ETC - I would recommend getting a straight black background for the tank. You can buy this at your local fish store / petco / whatever - it's really just a giant sticker that covers the back. It will make the tank look exponentially better then without it. You could also do a photo based background (river, under the lake, whatever) if you want. Second thing I would recommend is an air pump if you can afford it. It's really not completely necessary but if you can put a bubbler in the tank it will add additional oxygen and will benefit the health of your fish.
4.
Location
Just throwing this in here. Be mindful of where you place your tank. Putting it right next to a window is bad - as it will create unnecessary algae growth. Having the black background will help this a bit but you should never really do it unless you have no choice. My 110 is in my bedroom and right in front of a window - mostly because I didn't have a choice as there was nowhere else to place it. It can be a pain because I have to scrape the algae off the sides more often BUT i did do my best to counteract it by putting a lot of algae eaters in the tank and not feeding them traditional food.
5.
Food
Really depends which type of fish you get. For community fish i'd recommend flake food. If you decide to get cichlids / larger fish I personally get my food from a site called aquabid.com in bulk from a seller called "UncleRick" (
AquaBid.com - Item # foodp1465168805 - **PRO-GRO* GROWTH & COLOR 1 FULL POUND! - Ends: Sun Jun 5 2016 - 06:20:05 PM CDT). Food is fantastic for growth and color - and by the weight it's incredibly cheaper then anything you'll find at a pet store. For any bottom feeders / plecos you can get algae wafers and sinking shrimp pellets (whatever is cheapest - I used to utilize amazon for this). I would really encourage you NOT to buy any live food for the fish - unless you need to use feeder fish for any larger fish... I would advise against these type of animals anyways as a beginner. Reason being is that the live food can carry parasites which will infect your pets. Stear clear of blood worms as well, i'll let you google that one on your own.
6.
Cleaning
Cleaning can be relatively simple. Get a vacuum cleaning tool and a bucket. You start the water flow and literally just vacuum the waste out of the gravel. If you have a green thumb / a garden the water from the tank can be awesome for plants. Then just refill it with tap water. Make sure to get a chlorine treatment solution - you can buy this at a petstore / amazon for fairly cheap. Cleaning should be done every 2-3 weeks.
7.
Cycling
You will need to cycle your tank. By cycle I mean set the tank up but do NOT add fish until it has been running for between 2-4 weeks. What this does is it allows the nitorgen cycle to complete to the point where there is no toxins in the water that they fish will have difficulty adjusting to. (
Fishkeeping - Fishless Tank Cycling and Avoiding New Tank Syndrome - General Guides - Articles)
Cichlids - I mentioned earlier i'd do a little write up on this. Mostly because they are my favorite type of fish to keep. They are without a doubt the most colorful and interesting fish you can buy (outside of saltwater). There are generally two kinds "Old World" and "New World" Cichlids - it is VERY important NEVER to mix the two in the same tank. New World Cichlids typically come from South America / Asia / Central America- some examples if you would like to google them are Green Terror's, Jack Dempsey's, Oscars, Convicts ,etc. Old World Cichlids are from Africa and are typically confined to a few different lakes - Lake Malawi, Tanganyika and Victoria. What makes these fish incredible cool is they have all evolved over the course of millions and millions of years from one single breed of fish in each of their individual lakes - the most famous of these is Malawi (and what you will typically see in the fish stores). What is really cool about these fish are their vibrant colors and unique mannerisms. Since they have been evolving in each specific location for so long they have created very unique body language which can be incredibly entertaining to watch. If you want to go the cichlid route I would really encourage you to look at these fish. I would recommend a minimum of 55 gallons and some decent filtration as they eat and crap a lot. African Cichlids come in three different categories: Mbuna (rock dwellers, they tend to have rounder heads and not much finnage - of the three categories these are the most aggressive), Haplochromines (or haps for short - these are the deep water cichlids and can get larger then the others, but are a bit more peaceful and tend to have more color and more developed fins), the last are the peacocks (males are incredibly colorful - think reds, pinks, blues, yellows, with beautiful fins and typically black stripes along their bodies). These are my personal favorite fish but can be a tad more expensive, although nothing crazy.
Ok end of my rant. Only fish I would steer you away from is anything that could get too large (Oscars, Red Tail Catfish, Arowana) or any fish that are not meant for beginners (discus, rare cichlids, etc). If I were you I would check out
www.reddit.com/r/Aquariums/ and check out some of the articles they have tagged for beginners. I have also been a huge fan of buying fish online through aquabid.com - if that isn't for you it is still a GREAT resource to get an idea of what you may want to stock your tank with, without having to go to the store.
If you have any specific questions feel free to shoot me a message. And remember, it's a fun hobby - make sure to create a really cool underwater landscape and have FUN!
(apologies in advance for any typos - i refuse to proofread this).
Cheers