First, check to make sure that you are not using any water.
Second, locate your water meter. It is usually located in your basement or crawlspace near the front of your house.
If you look closely at the water meter you will see either a small black triangle or a small red dial, depending on the meter manufacturer. If this dial is moving, there is water passing through the meter.
The most common cause of leaks in a home are toilets. Toilet leaks can be hard to locate because the water goes down the drain and sometimes make very little sound. The next most likely place is hose bibs or spigots. Make sure you check out all outside fixtures.
If you still cannot locate the problem feel free to schedule a customer service representative to visit your home and solve the problem for you.