I recommend buying a book to start off with, contentious marine aquariast is a good one. there are so many ways of setting up a marine aquarium, today i will cover 1 way.
Tank
Heater for tropical
hydrometer for SG lvl
protein skimmer, removes organic waste suspended in the water and removed nitrates and phophates etc,.
test kits : ammonia, nitrite, nitrate, phosphate, PH for reef you need others like Calcium, iodine, strontium.
Powerheads to make a turnover rate of atleast 30 x per hour unless you have seahorse or pipefish.
lights, reef = MH, T5, PC, compact fluroscent
Fowler = fluros, Compact fluroscent
Liverock and Live sand for biological filtration
Salt unless you collect natural sea water
RO/DI or deionizing unit for tap water purification, or access to rain water