I wouldn't have a 20 gallon as your first SW tank. They are much more difficult to maintain than a larger tank. Stablilizing water quality and conditions required daily attention, so if you can't provide this, wait until you can get a bigger tank.
If you feel up to this, the interanl filter wil lbe useless. You'll want to run it without any media, just for water movement. You'll want to have 4-6 inches of sand, and 25-35 pounds of live rock. You'll need a heater, a skimmer would be a good idea, since it will help maintain good water, and upgraded lighting for anything but fish, crabs, shrimps and sponges. The lighting probably isn't adequate for more--being 'pretty bright' doesn't say much. Wattage, output, etc. Not a stock thing, but I have seen the kits modified for a small reef. Check before you buy what type of lighting it is, what kind of bulbs you have, ect.
You're going to be limited to one, maybe two small fish. I would go with a chalk basslet, and a small pygmy species angel. OR, one percula and a small blenny. Add snails (2-3 of 2-3 species), hermit crabs(10-12) and shrimp(2-3). Making sure the tank is well established with micro fauna first will be help ensure the life and health of the fish.
Above all, go slow. Cycle the tank carefully, spend time learning how to maintain the salinity, ect. You'll benefit in the long run.
