If you have a good local fish store, that carries saltwater, hopefully they will be able to help you.. A sign of a good saltwater dealer is if they have healthy, live, corals and anenomes..
I would approach any good live saltwater store, and ask if you can buy a few lbs of live sand from them.. Don't bother with the pre-packaged live sand, as it usually is out-dated or dosn't work as well.. Add a few lbs of live sand to a new sand bed of aragonite. (Finely crushed coral).. Don't use crushed coral, or pooka shell substrate, as it will cause 'pockets' of harmful gas in your sandbed.
Purchase a good amount of live-rock, atleast a good 40 dollar chunk, and that will cycle your aquarium.. Get yourself a test kit, and watch the ammonia and nitrite rise, and soon after they will go down and you will be left with Nitrate. Often the Nitrate will also dis-appear because you will have de-nitrifying organisms living in your live-rock that will help eat nitrate. Often adding live sand and a healthy amount of live-rock will self-cycle a tank in a few weeks.
Add fish very slowly, as you are extremely limited with a 20g tank. Feed very sparingly at first, as you will need to let bacteria build up in order to break down fish waste. Ammonia is extremely toxic in a saltwater tank, so make sure you feed sparingly and don't let anything 'decay' in the tank. Keep in mind when working with a small tank, such as 20g, your water quality will go through changes alot more rapidly than if you were working with 50 gallons of water volume.
Hopefully that will be a good start, on your quest for information!
Miles