The administrator of the site/organization need to add you as a user first.
You will receive an email invitation containing an activation link. Click the link to accept the invitation. Check your spam folder or contact the administrator to add you again, if you don't receive the invitation.
You will be directed to the registration page.
Follow the instruction by clicking on "Next":
Accept "Terms & Conditions":
Set a password:
Log in into the OCP with your email address and password and start your OCP journey.