Fitness can be measured by the volume of oxygen you can consume while exercising at your maximum capacity. Vo2max is the maximum amount of oxygen in millilitres, one can use in one minute per kilogram of body weight. Those who are fit have higher Vo2max values and can exercise more intensely than those who are not as well conditioned.
The base premise of the Cooper test is to measure how far an athlete can run in 12 minutes. To undertake this test you will need a foot pod or other distance measuring device a flat area (no hills). The best is to the test at a local 400m track. Before doing the test you need to make sure you are warmed up. Once warmed up you start to run with the core objective of covering as much distance as possible within the 12 minute time frame. Record the distance to the nearest 100 meters then do a warm down.
The cooper test is a good benchmark of increasing your Vo2Max as you can do the test multiple times over a few months to get a feel if your Vo2Max is getting better.