substrate: sand or curshed coral, live rock is a great thing to have. some form of filtration, heater, test kit, thermomiter, for a fish only tank you won't need great lighting, so what ever comes the tank should be fine. salt, salinity tester. books... patience.
I use a fulval filter on my 29gallon, you can get a fuvlal... like 304 or 404, there are alot of differnt heaters to choose from. live rock will be one of the more expensive things, but once you understand how it does biological filtration and how important it is you'll realize its worth it. you can get play sand from homedepot.. only a certian kind, i'm not sure which kind it is... i bought mine from my lfs and spent more money on the same stuff. test kits are different prices, read posts and find out what ones people seem to use the most without problems... other than that just shop around and see what you NEED for the fish selection you make before buying anything.