First do not listen to ***** they are there to sell you stuff. Yes you can have a 20 gal salt tank but a doubt that the kit you're buying will be what you need. More than likly it is for freshwater and the lights and filtration will not be enough for saltwater needs. You will need a skimmer.
First fiquire out what you want to keep Fish only with live rock and live sand (FOWLR) or a reef tank. The more water volume in a SW tank the better. If you can go bigger do it.
I would suggest holding off and read read read. The Conscientious marine aquarists by Fenner is a great book to start with and of course these boards are a wonderful resource.
When you are ready look for a preowned set up in the newspaper or craigs list. There are some great deals out there if you are patient. Peolpe ungrading to a larger tank or ones that just jumped in and have lost to much money and want to get out.
This is a hobby that can be very rewarding but it can also drain your wallet and kill critters unnessasarily. Do your homework first and learn from other mistakes and take your time!
Welcome to the boards! SLOW DOWN AND BE PATIENT!