To design this experiment you must first pick an activity. It is a good idea to pick an activity which would be easy to measure water usage. For example, you could pick showering, however it might be difficult in the average house to measure the water usage in a shower. On the other hand, if you were to pick brushing your teeth, you could use a bucket to catch your water usage to measure later.
Second, you must to background research. Ask questions about how much water usage is used by the average American household. Break it down further by households with the same number of members as yours and further by the number of people your age. If you can further analyize national data to figure out what the average use for just brushing teeth would be even more useful. If you can't find specific data, you will need to get enough information to calcuate the average use for teeth brushing.
The capturing of data for your own usage is the easiest part. Simple collect your water usage each time you brush your teeth for a specific amount of time: a week, a month, etc. Count the number of times you brush during that time. Calculate your average water usage per time. You can then compare your usage to that you found in your basic research.