Because your tank is small, I think you really only have to test the ammonia. I've heard some people say you really don't need
to test anything but I think it's a good idea. I have a small $4 test kit for ammonia that will last me a while for my 2 gallon. The less ammonia, the better. So .0ppm is the best for your betta and I'd say anywhere above .50ppm, you should preform a water change.
Also, you should check out the "water chemistry basics" sticky. http://www.bettafish.com/showthread.php?t=66595