I'm not sure of your experience level, so if you know some of this, sorry. First off, make sure your tank is completely cycled before you stock anything. Read up on this. You can use some live rock to cycle the tank, it will take 3-4 weeks. I'd reccommend getting "The Concientious Marine Aquarist " by Fenner. A very good book with much important info on set up, fish and inverts, everything you'll need really.
While the tank cycles, read about the fish and inverts you want, make sure they are compatible with each other. Go slowly, take your time stocking the fish, you'll have less problems if you're patient.
For this size tank, I'd only add 2 fish, at least for the first six months or so. The clownfish is a nice choice. Maybe try and get a mated pair.
As an idea for a 20 gl tank: 15 lbs live rock, two clownfish (small, same size and same species is important here...one will become dominant and turn female), a couple of turbo snails, 2-3 red leg hermits, and a coral banded shrimp.....
Does this help, or have I confused you?