I made two mistakes when I first started (yeah right, only two):
1. UGF filter
2. Crushed coral
I've since capped off the ugf and am still removing the crushed coral and replacing with sugar size sand...slowly. Got my first batch of info from a lfs. Got the correct batch of info from here on the forums. You're ahead of the game by asking these guys 'cause you can avoid a lot of problems. When the fish store suggests something, I check here before I buy.
Good luck,
Tom