Your web host should provide you with access statistics. This can be a bit difficult to decipher at first - many people mistake file requests for individual visitors and suddenly they are under the impression that they have millions of hits per month.
If your web host supports PHP there is a fairly easy to use script that monitors your site traffic and there are plenty of third party applications that do the same.
I'd have a look at your web host to begin with though. Most hosts offer statistics in one form or another.