Firstly click on 'my account' on the top right and than on the next page select affiliate settings. Than insert your Google Adsense code under the 'affiliate code section'
- my account --> affiliate settings --> Insert you adsense code
If you don't have an account with Google Adsense than you need to go to www.google.com/adsense and sign up for an account there.
I also recommend you get an amazon associates account.