Have you cycled your tank yet, if not some good information about it is here: http://www.fnzas.org.nz/?p=1868
I personally dont cycle with live fish, so I would cycle the tank, then putt in the fish that I want to have.
It really depends on what you want the tank to look like, personally I say plant the tank up, and put in a schooling fish like tetras (neon, ember, etc), or you could go guppies. Other then that you could get a school of dwarf rainbow fish, or put in dwarf gouramie. if you want a cat fish, corry cat fish are always cute to have.