Hello and welcome! You've come to the right place.
First you need to buy some test kits, if you haven't already. You will need a refractomer or hydrometer to measure salinity (refractometers are very much preferred), and test kits for pH, alkalinity, ammonia, nitrite and nitrate.
Then you will need to add something in the tank to start a cycle -- a piece of raw shrimp from the grocery store is ideal. Just drop it in the tank and let it rot. Never cycle using a live fish.
Then test your water every few days. You'll start to see ammonia rise, then nitrites. Then ammonia and nitrites will drop as nitrates rise. When ammonia and nitrites are both at zero, the tank has finished cycling. Do a water change to get the nitrates down to less than 20 (the closer to zero the better) and then you are set to add your first fish and clean up crew!
Be sure to only add one fish every 3 or 4 weeks. Or two at a time, if you buy a pair of the same species. Adding too many fish too fast is a surefire way to crash your tank.