Google Ad Manager is a program offered by Google to help website owners manage their own ad inventory.

To get started you need a Google AdSense that count. If you don’t have one, you can apply for it directly from the Google ad manager site. This could take up to one week for you to be approved. 

One way to speed the process up is to start a Blogger.com blog and apply for Adsense then as well. If you already have an Adsense account check the yes button.