Delete/Cancel my account

Here are the directions for canceling GApps for you domain:
http://www.google.com/support/a/bin/answer.py?answer=56236&query=cancel+account&topic=&type=

If you purchased your domain name from Google while signing up for GApps then you must transfer your domain name to a Domain Registrar of your choosing. The receiving Registrar will give you the steps to use but you must log into the Domain Registrar set up by Google for you - either GoDaddy or eNom. You will want to get the Authorization code from GoDaddy or eNom and unlock your domain. You may transfer from GoDaddy(Google account) to GoDaddy(personal account) using this process. DO NOT attempt to delete your account until you have transfered your domain name.

You may be limited by ICANN rules and can't transfer an account for 60 days after its creation or last move.