The CPU comes with a fan, but they're not the best. You can use that for a little while and if temps are not to your liking, you can get an aftermarket cooler.
Your Antec 900 should come with two fans in the front, one in the rear, and one on the top. So you'll be good there. I'd get a better case than the Antec 900 though. That's an old case.
Thermal paste is always a good idea. The stock cooler has a little on it, but its not good. Wipe it off and use some Arctic Silver 5 instead.