Many people on this site recommend the API Freshwater Master Test Kit. It tests for ammonia, nitrites, nitrates, PH and high PH. You should be able to find it online for about $25, though here in Australia it can be expensive - my LFS quoted me $80 yesterday, but I found an Aussie online source selling for $35. Shop around. But whatever you do, avoid the test strips. They can be very inaccurate if not stored and used exactly right. Buy a liquid test kit.