Welcome to the
Salty Side 
Some additionals things you will need
Salt for when you do water changes
test kits first ammonia, nitrite and nitrate and then maybe pH, calcium and alkalinty depending upon what you keep
salinity testing I usually recommend a refractometer since a hydrometer can be very inaccurate
make up water since salt does not evaporate, most of the time you will be topping off with fresh water. Since you want to start with pure water and since the contents of tap water can vary, it is usually recommended to get RO/DI water
live rock since that can provide the majority of filtration in the tank and looks natural. You will need around 1 lb per gallon depending upon your stocking plans
substrate I like the look of a sand bed and prefer a deep sand bed for the ability of it to process nitrate into nitrogen gas. I also like the critters I have to keep the sand bed clean. Some people go with a bare bottom thus increasing the flow. A crushed coral substrate, IMHO, is to be avoided since it increases the work load since it needs to be kept clean to avoid it becoming a nitrate factory
power heads - several power heads placed around the tank to increase circulation and to eliminate dead spots. For example in a reef tank you want to aim over 10x the water volume so if you are going reef, you need a turnover rate of over 450 gallons per hour
protein skimmer not required but I usually recommend one
What are your stocking plans? Fish only? Fish and some other critters such as snails, crabs, etc? Reef tank that has corals in it? It would help to narrow down the list of what you will if you kind of knew what you wanted to stock in there. But beaware, a lot of us, myself included
ONLY wanted a fish only tank but ended up going reef. Since I went that route, I ended up throwing out some stuff I bought that was not good for a reef tank.
