Well there is a lot you will need lol
A skimmer (depending on how big your tank is as you may get away with 10-20% water changes weekly)
A refractometer or hydrometer to measure sal content (refractometers are better)
powerheads to provide water movement
tests kits (PH, Nitrite, Nitrate and Ammonia calcium if you want to keep corals)
maybe a sump or an external canister filter (depending on tank size)