The WordAds program has a minimum traffic requirement before your site can be accepted. Once you have reached that amount your site will be automatically accepted in.
Alternatively, if you purchase our premium subscription you will be automatically accepted into WordAds.
If you decide to go that route, once you have paid for Premium go to WordPress.com/plans/my-plan
Scroll down to the bottom, there will be a button labeled Start Earning, click on that to join WordAds.
Fill out your info and you will be all set.
If you have any additional questions please feel free to ask.