The link Shawnie gave is going to give you great advice. If I read your post right, you've already got a betta in the tank. If this is correct, your fish will want you to do partial water changes every day to keep
ammonia/
nitrite levels down while the tank cycles.
I would also invest in the
API Master
test kit to do your own testing. Most fish stores use the little test strips because they're faster. Unfortunately, they're also not so accurate. Much better to do your own tests. The test kit runs from $15 to $25 (depending on who's selling it), but it lasts forever, and once you run out of a particular test, you just have to replace that test, not the whole kit, because API sells the individual tests, as well.