There are many different rands of salt on the market, all of them being basically the same. The only difference among them is whether or not they have nitrates and phosphates. Both of these are very bad for aquaria, so salts which have them must be avoided. Good salts include Instant Ocean (IO), IO Reef Crystals, and Coralife. As a note, standard rock salt can not be used as a substitute for marine salt mixes. Rock salt does not contain the important elements that marine creatures need to survive.
To measure the specific gravity of your saltwater you will need a hydrometer. There are two basic types of hydrometers available to hobbyist, the floating kind which usually measures temperature as well, and the plastic kind with a floating arm. It's basically a toss up as to which one to get, but the plastic kind has a larger scale and is easier to read.
