To add Google AdSense you first need to have your own website and host on a public host server. Then you need a Google login and password. You can use your Gmail account username and password if you have one. Otherwise you will need to register with Google AdSense. To go to Google Adsense click here. (http://www.google.com/adsense)
So What is Google AdSense? Google.com earns most of its revenue by allowing other website owners to advertise on their search result pages. All this is managed through a program they call AdWords. Now you can earn a share of the revenue that Google earns from AdWords by displaying these same text ads on your site. In other words, you're helping Google advertise and they pay you a percentage of what they earn.
To create Adsense or Google ads on your website login to your Google AdSense account and follow the steps given by Google. If you want more details in creating you AdSense click here.