I use the Roku Stick and it gets the job done. Can be sluggish at times, I hear the Roku 3 is alot smoothers.
1. makes sure your internet service is quick.
2. yes you need some kind of device like the ones mentioned above (I too would recommend Roku).
3. Like HuskyHawk said before careful with USB sticks, if its in an awkard place it may not fit (mine barely fits its in the back of my tv stick out in the direction of the wall. Its mainly a problem for me because my tv is mounted to the wall).
4. If you are getting rid of cable altogether, yes i would recommend getting a digital antenna, give you channels for free.
5. If you are doing it to get a lower rate than you do with cable, make sure you arent adding on a bunch of things that make it just as expensive if not more (if you get Amazon, and Hulu, and Netflix, and Sling Tv, ect... it adds up really quickly).
6. Make sure the services you are going to have streaming services with cover all the shows you feel like you cannot live without. When you find out you cant watch one of your fav shows anymore when you are sitting down and ready for action, you'll be pissed off.
7. Like mentioned above, alot of Smart TVs come with built in apps, that can save you some money on steaming devices. 1 of my tvs has a roku, the other has built in apps that work for the room its in.
Thats all i can think of.