20 gallon saltwater help?

I have owned freshwater fish all my life, but I wanted to learn how to take care of saltwater fish. I know the stocking should be 1" per 5 gallons; however since I'm a new at this I was wondering if anyone can help me set this up. I plan to buy a Marineland HOT mag. hang on canister filter, Instant Ocean salt, and 20lb bag of Bioactive sand. I also plan to have live rock and coral. How many lbs I can put in that size tank? I was planning on buying a clownfish, a small goby and a sm blue damsel. Could I also be able to fit a starfish and a hermit crab? Any advice would be awesome. FYI I'm in college so I do not plan to spend more then $250 (I already have a tank/stand) so I can't afford really expensive equipment. Thanks