Google analytics is in our opinion the best option for keeping track of your visitor statistics. Other tracking tools will also work in a similar way, but we only supply addons to view the stats in the dashboard for google analytics.
Here is our guide to getting started.
To get google analytics to start collecting your website stats first thing you need to do is sign up to their service: http://www.google.com/analytics/ - you should not require a premium service
Sign in to your Analytics account, and go to the admin page. Select an account from the dropdown in the Account column. Select a property from the dropdown in the Property column. Click Tracking Info.
Head to your theme preferences area (found in the appearance menu), click the advanced options and then paste your tracking code in the custom headers box
This code should look something along the lines of this:
You can view your stats on the google analytics website as well as being able to download their apps for your mobile phone.