When I first began putting my aquarium together I had the same exact problem!
I would suggest two things:

1. Shop at petsmart, they have a 14 day policy that you can return the fish (dead or alive) if you don't like it.

2. Set up a quarantine tank so that you can observe the fish and put it in with the others once you are sure they are healthy.

Good luck!!! (from a fellow fishkeeper!) :)