As a beginner fishkeeper you may or may not have been informed of these at the time of purchase with your tank. Water chemistry test kits are a necessity as a beginner, for two main reasons. One being: It allows you to monitor the cycle process within your tank, and take corrective action when needed, as to not lose any fish unnecessarily. Two being: It almost forces education on these water parameters and their importance within your new underwater world.
Most test kits are sold individually at your LFS (Local Fish Store) as well as what is called "A Master Test Kit" which contains the four main test kits needed, as well as a couple more needed later on after the cycle has completed.
The three main tests you'll need to start your new tank are:
PH - Which is the measure of alkaline vs. acidity
Ammonia - Which is what is excreted by fish via gills, and waste (pee and poo). This is also the food necessary for bacteria (nitrosomas)to begin growing (see the Nitrogen Cycle section )these types of bacteria will breakdown the ammonia into....
Nitrite - This is turn breaks down by bacteria called nitrobacters and turns the nitrite into....
Nitrate - This is the end result of the Nitrogen cycle, and is relatively harmless in low concentrations, and is measured in ppms (parts per million) By measuring these changes within your tank, it allows you to see the process of the cycle more clearly as well as understand it. It is also recommended that you get a "gravel vacuum hose" at the same time, for if levels become too toxic for you fish, you will need to do a water change to reduce the levels before fish loss may occur. There are other test kits as well that will measure anything from hardness of your water to phosphate level, these test are useful in troubleshooting later on within your tank, but not necessarily of importance during the cycle period. By getting these tests as soon as possible as well as a gravel vac., it allows not only you, but us as your fellow fishkeepers, to better assist you with this hobby as well as troubleshoot any problems that may occur.