Welcome aboard, always nice to have new reefers. The first thing is your cycle. It's not really 3 weeks, it's really over when your test kits confirm that everything is within normal parameters. Once thats done then you can pretty much start to add fish and coral.
I would start with one or two "SMALL" fish and let your bio filter get established. A coral won't really do that for you.
A 20 won't take very much livestock so choose carefully. If you add too much too soon you're either going to get a crash or the nicest crop of hair algae you've ever seen
A nice site to look at fish and coral requirements is below. Have fun
http://www.liveaquaria.com/