well, since I've got nothing to do, I guess I'll give you a good explanation, although you really should go to the articles section and read up.
I'm assumng that since you have the tank, im guessin it was origionally a FW
tank. meaning you should already have the heater, but other than that, you really can't salvage anything else for it.
There are a few types of saltwater tanks. there is FO
(Fish Only), which is just fish, with no live rocks or coral or anything. There is FOWLR
(Fish Only With Live Rock), which is pretty self explanatory. Then, there are reef tanks, these are the most saught after taks. they have corals, and fish, and live rocks, but they are also the most expensive, and need some special equipment. Assuming by what you are saying , you want a reef tank, with the live rock, and the fish, and corals.
These are just a few of the things that you are going to need:
-HEATER: (just in case you don't already have one) for this size tank, I would get a 250+ watt heater (get two if you can, always good to have an extra for backup, in case one breaks, and for those cold winter months)
-SALT: some good ones are Instant Ocean, Reef Crystals Reef Salt, and Oceanic (although you are going to want to get more opinions on this one, im not a salt expert)
-REFRACTOMETER: This is an item that is used to measure the salt content of the water in your tank. You can also get an item called a hydrometer, which is cheaper, but I would HIGHLY recomend the refractometer, which is so
much more accurate and reliable.
-TEST KITS: kinda self explanatory. Tests the aquarium water for different things, like nitrates, nitrites, Ph, ammonia, and such. I'm sure you have purchased test kits before, but I'm still going to recommend that you go withg the API test kits.
-SUBSTRATE: Whatever is placed at the bottom of the tank, can be crushed rock, or sand. I would recommend sand, but it all just depends on what look you are going for. Oh, and whatever you do, don't buy "live" (bagged) sand. It is more expensive than regular sand, and anything that might have been live on the sand is now long dead. but that being said, I purchased live sand straight from a tank that I knew to be a well taken care of established tank.
-NET: For catching your little fishies. You should already have one.
-BUCKET: Needs WILL arise
-PROTIEN SKIMMER: I dont know if you need one quite yet, but you will need one at some point, so plan on buying one.
-POWERHEADS: get Koralia's
-LIGHTING: what kind of lighting that you get will all depend on what kind of tank you want.
Try, if possible, to get anything you can off of Craigslist or eBay, because this can be a very expensive hobby. Also, you can DIY pretty much everything in your tank.