Hey, I'm 15 and a reefer too. I'll give you a basic rundown of what you need, because you LFS sucks. Don't ever take their advice on anything. I never do.
In a 20, for filtration, you need:
-2 powerheads, at least, for the tank size.
-Enough LR. You want about 1.5-2lbs per gallon. For example, my 40G has around 70lbs.
-Water changes, every week to every other week.
You don't need a skimmer or a sump in a tank that size if you do your WC's. Your filtration is powerheads, LR/LS, and WC's.
Also, how experienced are you in fish keeping? If you have very little experience, you should go FW. Even if you aren't, you need to spend months and months of researching SW and how it runs.