Please go to every forum here (particularly the pool chemistry and testing forums) and read the stickied threads at the top of each one. We have myriad threads covering this very topic. Indeed, that's a large part of the essence of PoolForum.com--that we do our own water testing and get most of our pool chemicals at the grocery and hardware stores.

Knowledge/information will save you a bundle of $$$, and you'll be amazed at how much less time you spend on pool care, and how much more effective it is!