Google Adsense

First you must apply at Google to be able to add Google ads to your site.
After you have applied Google will get back to you and tell you whether your site meets Google's requirements. If it doesn't, Google will most likely tell you what you will have to do before you can reapply.

After your site has been approved you can start to create ads that you can add on your site.
And since you have been approved once, Google trusts that if you want to add ads to a new site you don't have apply for the new site since you know what it takes to meet Google's requirements.