Go to AO website www.aofoundation.org and click Login button.
Click Create new account. Click Use e-mail, type in your primary e-mail address, and follow the process.
After all fields has been filled in, a summary will be shown. Check the summary, read and accept our terms and conditions/data protection policy, and click Register. You will receive a confirmation and will need to verify your e-mail address to confirm your registration within 48 hours. Without a verification of your registered e-mail address within 48 hours, no AO account will be created.
A step-by-step guide can be accessed here (login required): How to register a new AO account