You may add the Google Analytics tracking code to you site either manually or with a plugin. There are many plugins that can help you do this, but I have selected a very simple one, that get the job done simple and sweet.
Adding Google Analytics manually
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There are many plugins that allow you to manage your HTML header and help insert tags for Google, MSN and Yahoo webmaster, BlogCAtalog and many other services that require adding some tag to the header of your HTML.