Firstly, you'll need to set up a Google Analytics account. You can find instructions on how to do so here: https://support.google.com/analytics/answer/1008080?hl=en.
Now that you've set up your Analytics account you can integrate it with your uSkinned theme in three very quick steps :-).
Go to your Google Analytics account > Admin > Expand ".JS Tracking Info" > Click on Tracking Code (see screenshot below).
Now you should take a copy of the Tracking Code.
Now login to Umbraco CMS > Go to the Content section > Expand Website Configuration > Click on Global Settings > Go to the Scripts tab > Paste your code into the Headers Scripts field > Save and publish
That's it. You can now go to your Google Analytics dashboard to start monitoring your website visitors in real time and run reports on user behaviour. Enjoy!