I agree with the above advice, research and research more. You will need a lot of things, from rock and sand to filtration. But before I could offer you much help, what do you want to do with the tank? Is this going to be a FOWLR or a full blown reef? Is your tank drilled or non-drilled? What kind of live stock do you want to keep? The answers to these questions determine what you need. As far as basics that are the same for all tanks, here's your list.
1) You need sand and rock.
www.reefrocks.net You can not beat his prices, your going to need to decide how deep of a sand bed you want and decide how much to purchase from there. Only use aragonite sand, it provides benefits such as buffering ph. For rock, you need atleast 1-2lbs. per gallon depending on how porous the rock is. You can choose to use live rock, but I prefer to start dry to avoid pests.
2) You need some kind of filtration. If the tank is drilled or you want to use an HOB overflow, that means a sump/fuge. If it's not drilled and you do not wish to use an HOB overflow, your going to need some kind of mechanical filtration, but your going to have to keep up with maintenance more if you go this route.
3) You need to decide if you want to purchase or make your own RODI water. If you choose to buy from your LFS, you need buckets and lids. If you want to make it, your going to want a RODI Unit. Bulk Reef Supply makes a nice unit that is affordable.
4) Salt, kind of obvious but you need to decide what kind you want. I'd buy the buckets, they costs more up fron, but save you money in the long run. If you are going to do a reef, get reef salt as it provides more calcium and trace elements for coral growth.
5) You already have lights and powerheads, but you have to make sure they suit your needs. If your doing a reef, you need to make sure your lights put out enough PAR to support corals, and you need to make sure your powerheads create enough flow to keep a reef.
6) Protein Skimmer, get as large of a skimmer as practical for your particular system. I'd recommend trying to double your tank volume, so in your case look for a skimmer rated at 150+gallons. Stick to name brands on this, cheap skimmers will come back to bite you. Reef Octopus is a good affordable brand as is Bubble Magus. If your on a huge budget you can splurge for a Bubble King, though some of them cost more than my entire system, lol.
7) Testing supplies, such as an API test kit. They have basic kits and kits for reefs, which you need depends on your tank. You will also need a refractometer to measure salinity. Hydrometers will do in a pinch, but they are wildly inaccurate.
These are all essentials for a successful sw tank. What else you need depends entirely on your wants. If you want a full blown reef, you may want dosing systems, reactors, tank controllers, and about a million other optional add ons. The tank can be as simple or complex as you want it to be. Feel free to ask any questions you have as your going along, good luck!
