(Note: If you are looking to cancel your service, know that AT&T has a Early Termination Fee (ETF) that is almost unavoidable. In order to avoid the fee, you should target the end of a billing cycle as you will still be paying for the remaining time until the end of the contract or pay period. This cannot be done online as this can only be done on the phone.)
Step 1: AT&T doesn't let you cancel your service on their website. Unfortunately you'll need to call them.