Once you’ve clicked the “Activate my account” link in your welcome email, you’ll be taken to the Set your password page:
If you move your cursor over the question mark to the right of the password field, you’ll see the password requirements:
A password needs to have at least 8 characters, at least one lower case, at least 1 upper case, at least one digit and at least one non-alphanumeric character.
Type in your new password again to confirm it, then click the Activate account button.
If everything went well, you should get an Account Activated message, and clicking Close should take you to the normal login screen.