Before you purchase any livestock, do some reading/research on saltwater aquariums, fish and inverts. It will help you enjoy the experience a lot more.
Stay away from tangs, angels, butterflies, triggers, groupers, large lionfish, large puffers. They either need larger tanks or do best in very well aged systems.
Good beginner fish; clowns, dartfish, basslets, chromis, cardinals, wrasses.
Good book selections include The Conscientious Marine Aquarist by Robert Fenner, and The New Marine Aquarium by Michael Paletta. Also pick up a copy of 500 Marine Aquarium Fish by Scott Michael.