Its quite easy. Go to My Account> Earnings> Click Configure on Google AdSense. If you have an approved Google AdSense account, then click on Yes and enter your postal code and last 5 digits of your phone number. HubPages and Google will automatically configure your account and link it with HubPages. Once linked, your Google AdSense publisher ID will be shown next to Google AdSense in the above Earnings tab. Now the final step you have to do is to check you mail and accept the configuration for HubPages.
If you don't have an account with AdSense, then click on No and follow the steps to create an account. But keep in mind that you should have atleast 10 quality hubs before applying for AdSense else you might get rejected. Also, write a decent profile about yourself with a good profile picture. All the best!