If you are pretty new to the hobby, Instant Ocean is probably the best place to start.
Like mentioned above, it's cheap. And it's easy to find.
Once you are in the hobby for a while and you learn your tank and have coral that are using up various amounts of Calc and Alk you might want to start playing with some other salts that have different amounts of these elements in them, or seeing if your tank responds differently to different salts.
Personally, I really like AquaVitro Salinity. It never clumps/hardens, mixes pretty clean, it mixes up to 1.025 which most salts do not... more around 1.022 so that makes it not as expensive as it initially seems, I love the bucket it comes in, and my tank just seems to like it.