The one thing everyone will tell you before you should buy or attempt anything... do your research! Trust me, do not attempt to buy a single thing until you know what you're doing and why. Educate yourself until you know the basic fundamentals. See what a SW tank requires and so on.
I notice you're converting, so you need to make sure your tank is completely clean and sterile. There is a few Youtube video's on how to clean a tank out properly for SW conversions.
Do not buy any sand or rock until you have the necessary equipment.
For a 75G, you're going to need a Protein Skimmer (that is a MUST). A sump would be ideal as you wouldn't want a big protein skimmer hanging off your tank, unless you get a nano one which may not be ideal for a tank that size, espesh if you're keeping corals. You NEED powerheads. Again, research what sort of flow you need and the appropriate powerheads. Hydor Koralia's are good ones. You should have a heater, a good one at that. If you're wanting to keep corals, you need appropriate lighting, too. This doesn't come too cheap but if you have the fittings already, it shouldn't be too much of a problem.
Once you have the correct equipment in place, you should do a clean run with R/O ideally. Let the system wash itself out and empty and re-fill. Twice would be great, but once would do. Do not use regular tap water and then dechlorinate it. You can get away with that in FW tanks, but the water will still contain various chemicals and other stuff which you do not want, so unless you have it already, you'll need an RO/DI system, unless you wish to have half a dozen tips to the LFS for RO (You don't want to do this, it's too costly and a big pain). From there, it's just a matter of purchasing a good brand of salt and adding that until your salinity is between 1.023-1.025 (You'll need a refractometer for this).
It's all about just doing your homework. Really, don't go into it so fast if you're not sure of the necessary equipment and fundamentals of keeping a SW tank, because it's a completely different ball game compared to FW tanks.