Hi and welcome to AA!
Best thing to do is start researching what fish you want to keep. Liveaquaria.com is a good place for that. It'll give you the tank size requirements of the fish you want etc.
Also a great place to start is by reading the articles here on AA:
Make sure to read about cycling you tank. I'd suggest a fishless cycle using pure ammonia. It's faster, and easier than other methods in my opinion.
Honestly, just read anything and everything. Feel free to ask questions along the way!.