First off I would read everything there is to read on Marine (SW tanks). Or at least enough to know how to set up and maintaine a fish only system and able to anwser questions like:
What does a protien skimmer remove from the water? and why doesnt it work in fresh water aquariums?
And you will also soon get to know which fish are compatanble with what an what requirements each needs to live. I.e. a Yellow tang needs 120cm (length) tank to swim comfortably.
If you are after a smaller aquarium you could well want clownfish, damselfish or dwarf angelfish.