You just need to sign up for AdSense. Then, when you've been accepted into AdSense, just participate in the HubPages earnings program. Here are some links from Hubpages to help you get started: https://hubpageshelp.com/money/Learning-Center-sig and https://hubpageshelp.com/money/Learning-Center-hub... . You can also check out the HubPages FAQ for more information. Here's the link: https://hubpages.com/faq/ .
Some things to remember - if you're signing up to AdSense via HubPages, HubPages suggests that you have at least 10 hubs before applying for AdSense. However, since many have been rejected for insufficient content even with 10 hubs, you may opt to apply for AdSense when you have way more hubs than that to decrease the chances of you being rejected.
Be sure to read up on all of the AdSense terms of service, FAQ, policies, guidelines, rules, etc to make sure that you're not inadvertently violating anything as that can also hinder your AdSense approval. Hope this helped.