That's a very broad question. You going for fish only? Reef only? Reef with fish? SPS, LPS, softies, anemones? All of these things will make your equipment requirements very different. If you go fish only then the lights you
Use will make no difference. Certain corals need different light intensities so depending on that you might want LEDs or halides or even compact fluorescents. You will need a heavier flow then a normal freshwater tank. You will want your pump or filter be able to turn over all the water in your tank about 10x in volume per hour (so a 20 gallon tank you will want a pump that will give you 200 gallons an hour, but keep in mind that if you have a sump that you will need a more powerful one due to head loss) you will need a good clean up crew (provided that you don't get fish that would eat them) a mix of hermit crabs and snails. A power head is always a good idea too. Helps keep water movement and reduces dead zones.
Would really help US give YOU advice if you had an idea of what you want to put in there. Helps us help you choose tankmates and equipment to stock your tank with.