You will need your own domain name. It is a requirement from Google.
If you don't have a name, 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 HTML code to place on your site. Copy the reference number that comes immediately after "ca-pub-".
Go to Site Settings and find "Set Ads". You will see a field ca-pub-", paste the reference number here.
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.