If I were you the first thing I would consider is a bigger tank for a few reasons. One being a 20 gallon tank is going to be pretty hard to keep a good water quality and two being most fish that make good first fish will need more room than 20 gallons. Most literature recomends at least 30 gallons for a salt water set up. If you already have a 20 gallon tank you could use that as a quarantine tank or easily make a sump (wet/dry) filter out of it. I would most definatly encourage a quarintine tank though and the basics to maintain that tank, (filter, air pump, heater, and thermometer.) For your display tank I would go with a wet/dry or canister filter, again you can easily make your own wet/dry filter. A few power heads, heater, thermometer, live rock,(the more the better), substrate, hydrometer or if you are feeling spendy a refractomeater, testing supplies, salt, adequate lighting, and most of all lots of time and reserch. Make sure you know what you are getting into before you jump the gun.