We provide an excellent website statistics engine that provides statistics on visitors to your website. This is run on the PIWIK platform.
Many users also like to utilise Google Analytics, an excellent free tool for all visitor statistics.
To start using the system you'll need to register with Google, http://analytics.google.com and then paste the code they provide you with into the box provided from the administrator's toolbar (Website settings > Web stats). The code will immediately be added to your website after you hit save, however it may not actually appear on Google's site for up to 24 hours.