There are two ways to do this.
a) Use a third-party service (many can be found on Google)
b) Host one on your website.

The latter bears a bit of explanation. Your web host will have to support PHP, Perl, or some other server-side scripting language. Provided it does, a hit counter can be written in just 5 or 10 lines of code. I can post code if you want.

Hope it helps.