First off, Welcome to AquariumAdvice.com!!!
The very first thing you should buy when looking into SW
tanks is a good book about them. I like "The Conscienious Marine Aquarist" by Robert Fenner. A 20gal, while not ideal, will work for a first SW
tank. In a tank of that size, keeping water parameters consistant and acceptable is your biggest challenge. I second the idea of using LR
(30-40lbs), a 3in sand bed and a good skimmer for your filtration. With this you will not need a mechanical filter (like a fluval) but it would add low to the tank and give you a place to run chemical filtration (like carbon). Keep up the research and let us know how things go.