Generally an ammonia, nitrate and nitrite water test kit plays an important role when setting new tank when the beneficial bacteria has yet to develop. During that period, it is imperative to monitor closely on the level on daily basis so that any fluctuation can be easily detected. Once the tank has established, you can easily reduce the frequency to have it done at least once every two weeks.
When buying the test kit, make sure that the pack comes with all three indicators so that you can cover all parameters at once. Most aquarium stores will sell test kit either for ammonia, nitrite or nitrate but I would not recommend buying them individually. Usually the price for a complete set ranges from $5 to $20 depending on the quality and manufacturers. Using the test kit is very simple because all you need to do is to dip in the test strip and the resulting color can be compared with a a standard color scale. There is also another type which requires you to obtain a sample of aquarium water and then add in a few drops of test solution to get the color reading.
Finally, there are several actions which you can take if you find that the chemicals fluctuate above a certain level:
1) Perform water change
2) Rinse and check for clogged up aquarium filter