Well, to start off, you would need a bag of synthetic sea salt from your LFS, On the packaging, they will normally state what tank size its good for, so buy the number of bags of salt you need according to tank size (and you might wanna keep a couple at home just in case) and buy a hydrometer too.
Fill your empty tank up with either reverse osmosis or distilled water, add in salt slowly a bit at a time and make sure all the salt is dissolved, then check salinity, repeat this process until you get an ideal salinity ( 1.024 -1.026 ) once you're done, add in all your other stuff and leave it to cycle.
i was wanting to know because i just got a reef tank setup and i have to change the water biweekly and i usually have the luxury of just buying saltwater made up and i thought i needed to learn how to make it as well so in case of emergency so i have a backup plan.
The best way to make saltwater is to start with RO/DI water (maybe from your LFS). If you want to make 5 gallons or less, you can use a 7-gallon salt bucket (very useful to have some of those around). For larger quantities, most people use large Rubbermaid garbage cans with wheels.
Put the water in first, then add the required amount of salt. Stir until reasonably dissolved. Put in a powerhead or other pump (I suspend it near the bottom of the container). Leave bubbling overnight.
It's also important to test the water for sg and pH and make adjustments. You can make the first readings after you stir and dissolve the water.
It's possible to buy saltwater from your LFS, but then you need to test it and make any necessary adjustments, which usually involves adding salt.