Install Google Analytics on your site.
you can either use Google Analytics or StatCounter Free invisible Web tracker, Hit counter and Web stats
I can recommend two free hit counters. These are Statcounter and Bravenet. You get three free ones with each of these sites. And of course you can upgrade.
They are really good, reliable and easy to install. (BTW, Bravenet is just a counter - it's a hosting company with heaps of tools, including counters. You just have to sign up and find the right section in your membership.)
I use Clicky - definitely the best option. Its real time, isn't a cheap hit counter and provides advanced yet very easy to understand analytics. Its used by many of the big blogs