Aquarium salinity is one of the most deciding factors in keeping saltwater or brackish fish. Think of it as balancing out the oxygen we breath. Pure oxygen will actually kill a human. Likewise swimming in pure water will kill our marine fish.
To provide them with the optimal salinity it is best to look at their specific care sheet. If you have a tank with many fish or are unsure which you will be keeping it is safe to put the salinity at 1.026. This is the most commonly used number among marine tank hobbyists and by far one of the safest numbers. Should the water evaporate causing the salinity to raise you have a buffer zone before the water becomes too salty. This also works for topping off, adding un-salted water, as the salinity is high enough that a small drop will not harm your fish.
When choosing which salt to add to your aquarium there is one simple question you have to ask yourself. Am I interested in corals? If you are not you have both dodged one of the most expensive hobbies there is and can use any aquarium salt. Don't get me wrong I love my corals but they can be a bit costly. If you do want to grow corals be on the lookout for reef salt. Salt labeled as reef salt will posses many trace elements that corals need to survive. While you can grow corals using normal salts, the amount of additives you would have to buy to match what comes with the salt is not worth the time/cost.
Always mix salt before adding to inhabited water!
If you have never set up a saltwater aquarium I can guarantee the amount of salt needed to create the right salinity will boggle your mind. Even a small 10 gallon saltwater tank will need cups of salt.
To begin you will first need your treated water. I highly recommend using a reverse osmosis filter to create the water, but tap water that has been treated works just fine. If the tank is smaller, I'd say up to twenty gallons, you will be fine stirring the water with your hand. Scoop in salt slowly as you stir the water, allowing the salt to dissolve.
Generally speaking you will want to use a refractometer to get the closest results possible. If you do not have one on hand or cannot get one you may use the rule of thumb: 1/2 cup of salt per gallon of water. This will give your tank a perfectly livable environment but may not be the perfect salt level for your fish. If you plant on keeping a saltwater tank long term you will need a refractometer.
If your tank is large, say 30+ gallons then hand mixing the salt will be far too time consuming. When this situation comes around many hobbyists will use circulation pumps to mix the water for them. Some of the cheaper circulation pumps are under $10 and will save you plenty of time. This is the place to use those bargain pumps, as the noise they produce and longevity will not really come into play. These will stir you water for at least 30 minutes, adding on more time as the batch of water gets bigger.
In the reef tank you will no doubt need to add water. The more powerful lights that are associated with saltwater aquariums, combined with sumps, surface agitation and protein skimmers will no doubt lower you water level every day. When the time comes there is one thing you need to keep in mind.
Salt does not evaporate
That means you will need to add normal water into the tank and not salt water. If you were to add saltwater each time the salinity would climb and eventually harm your fish. For the best results try to top off(refill your tank) around every other day or even add an automatic top off system.
That being said do not simply pour in all the water in an instant. Slowly add the water to avoid distressing your fish. While getting hit with freshwater will not instantly kill them it can bother them or scare them. We want our fish to be as stress free as possible.