You are going to get alot of different opinions so it is best to give us some ideas you have and we can get an idea of what you want then adjust from there. Also since we dont know you and your previous experience give us some basic tank info like water parameters, how long has it been setup, was it cycled? Do you know what cycling a tank is? This will help us help you
Is your tank tank cycled and ready to do?If so different schools of tetra's are always nice, with a "centre peice" of some kind, and a bottom deweller or a few depending on the type. This would be nice and simple.