You will need your own domain name. It is a requirement from Google.
If you don't have a domain, you can purchase one from YorName by going to Site Settings and selecting 'Site Address - Domain name'.
From your Google AdSense account, Google will provide you with a Publisher ID. Copy the reference number that comes immediately after "ca-pub-", or after "pub-".
Google's help article provides 4 ways to find your Publisher ID : https://support.google.com/adsense/answer/105516
Go to Site Settings and enable "Use your own AdSense account". Paste your Publisher ID in the field next to "ca-pub-", Apply and then Publish your site.
Select the kind of ads you want on your site from inside your AdSense account. Bear in mind, some ads can be very invasive.