Check out the
saltwater aquarium setup page for a list of needed equipment. Glad to hear you're researching the topic before diving in, very wise. Figure out what you would like to keep and see if it'll do well in a 20 gallon tank. I'll warn you though that if you have success with the saltwater tank that you'll soon be wanting to upgrade to a much larger tank to keep some of the really cool species. I say go as big as possible up front. You'll be glad you did down the road and it'll increase your chances of success.
Take your time and enjoy the research.
Welcome to Fishlore - Mike