Once you have filled out your personal information and chosen a payment method, you will be asked to complete a tax form. You can either choose one of the available forms or start a tax questionnaire.
Once you have chosen your tax form, you will be asked to provide personal information, including a tax number. If you are not a U.S. person, please provide your country's equivalent tax number. If you are a U.S. person, please provide your SSN/EIN or ITIN.
See also: Which tax form should I fill out?