The most common cause of a high water bill is a toilet leak. These are not always easily detectable. Check the water level in the tank to make sure it is not dripping into the filler pipe. Another simple test is to apply 4-5 drops of food coloring into the tank, wait 15-20 minutes. If it turns the water in the bowl to that color, then there is a leak.
Other causes of high usage could be leaks such as dripping faucets, shower heads or broken pipes (internal or irrigation). Also, during the warmer months people tend to use more water for watering lawns, washing cars, filling pools, etc..