In a nutshell, you will need the tank, a stand, a heater, lights of some sort, depending on what you want to keep. Filtration, which can either be via live rock, a sump, a hang on or a canister. Lots of test kits, for things like Ammonia, Nitrite, Nitrate, Ph. If you want to go reef you should also get kits for Calcium, Alkalinity, Magnesium. A hydrometer is a good investment for measuring your salinity. Other things that are good but arent immediately needed are a good protein skimmer and a couple of powerheads for water movement