Wow...that is an open-ended question. You will want rock and possibly sand (some people do bare bottom). You might need a heater, depending on where you are. You will want a saltwater test kit (unless you are going to run to your LFS
a lot for testing). You can buy pre-mixed saltwater or make your own. To test salinity, you need at least a hydrometer, preferably a refractometer (more accurate). You will need something to move water around, so probably a power head or two. You do not want dead spots in the tank.
Are you doing a reef or FOWLR