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
Bother of these addons can be used at the same time.
Gives you a visitor graph on your homepage, and small individual post graphs on the post lists pages
More in-depth information including visitor regions as well as working with your mailchimp campaign to show you site activity after you've sent a campaign.