First of all you will need a Google account. If you already use any of the Google services (gmail, Orkut, Google Docs, Picasa, etc.) you have a Google account and just use the same data to register your site in Analytics.
First go to the Google Analytics website: https://www.google.com/analytics/
Enter your Google account login data. If you are from another planet and do not yet have a Google account click on the "Sign up now" link and proceed with your registration.
After that, you'll be taken to the Google Analytics sign-up page. Click the Subscribe button.
Now on the New Account Registration page you will be able to enter your personal data and agree to the user agreement.
After going through the three steps of this page, you will be taken to the last step which is the step where you pick up the Tracking Code.
On this page you need to pay attention to the block where your tracking code is displayed: copy this code and go to the source code of your site, paste it into the source code in your site's HTML just before the </ body> tag, At the end of your site. You need to do this before continuing. Without this step, Google will not be able to track your site.
After logging in to Control Panel, click Configuration >> Google Analytics >> Select Google Analytics Code Page Finished Purchase Success >> EDIT >> Enter Google Analytics code
Once you add the tracking code to your site, click the Continue button at the bottom of the page and you'll be taken to a page you can have a listing of all your sites registered in Analytics (yes, you can add more than one site) .
Now, just wait for Google to begin collecting your site visitors data, so when you click Preview Report you'll be able to start seeing your visitors' graphics, pageviews, and everything else about your site visitors.