I would recommend a refractometer over a hydrometer, tend to be more accurate. I was having some trouble initially keeping salinity stable using a swing arm hydrometer. A refractometer should cost about 30 dollars, definitely worth it IMO.
For fish and invertebrates you want your salinity to be at least around 1.023. If you plan on keeping corals you should probably keep it at 1.025 so you get some more of the calcium, alk, etc that comes in your salt mix.
A good salt is one of the most important factors. There are many different mixes with different parameters. Personally I am using Fritz Reef Pro Mix, and my corals seem to love it. Mixes clear quickly. Another good brand which people seem to like, is Red Sea "blue bucket" salt.
To get to your desired salinity, you will need to use the refractometer/hydrometer. Mix some in, see what you're at. Mix more in, until you get to your desired level. After your initially fill your tank, you should never add salt directly into the tank; mix it in buckets to the proper salinity first, to ensure everything remains stable. You will need to top-off the tank with fresh water as evaporation occurs, to keep the salinity stable. Stability is probably the most important thing in a saltwater aquarium.
