Where i paste tracking code in my website ?
From the User Guide: http://piwik.org/docs/installation/#configuring-piwik
Install the JavaScript Tracking Tag
Piwik will issue you with a JavaScript tag. This code must appear on every page that you want Piwik to analyze. We recommend that you put this code just before your tag at the bottom of your pages (or in a general footer file that is included at the bottom of all your pages).
(my emphasis)
Mine’s in a global footer (on an SMF forum).