There are a couple of reasons why your water bill is unusually high. The number one reason for most customers with a high bill is a leaking toilet. This is the worst cause for a high water bill! You will need to check for the following things or call a plumber to do it for you.
If you hear what sounds like your toilet flushing all the time. (sometimes this can be a very faint running water sound) the water in the back tank is not stopping below the overflow. This means that the water level is too high and is constantly filling up. To stop this, gently bend the rod down. (Note: the rod has the ball attached to it, and some newer toilets may no longer have this ball system.)
In the back tank, there is a flapper. If the flapper doesn't sit properly after flushing, this could mean that it is either old or cracked. You must fix this immediately because the water is bypassing the flapper, which causes your toilet to waste a lot of water. Unlike the previous problem, you may not be able to hear this happening, so it is wise to check for this problem periodically.
On your water meter there is a leak detector. When looking at the top of your meter, you will see a small red triangle. While there is no water being used in the house, watch this triangle for a few minutes. If it is moving at all, then you have a leak somewhere in your house, drawing water through the meter.