Most CoLab sites are "closed"—meaning that accounts are by invitation only. Ordinarily, such invitations are sent by email with a link for you to confirm your interest and to set a password, as well as to fill out additional profile information so that other CoLab members can get to know you virtually.
In some cases, however, you may be able to create your own account, pending approval by one of the site moderators. To create an account, visit the login screen of a particular CoLab site and click the "Create new account" link (1):
From there, you'll be taken to a form in which you'll enter a few pieces of information so that the site moderator can evaluate your request and confirm your new account. At a minimum, you'll be asked for your email address (2), first name (3), and last name (4), but it's possible that a particular CoLab site may require additional details:
Once you click the blue "Create new account" button you'll receive an email confirmation of your submission. Once a site moderator has approved your account, you'll receive a second follow-up email with a one-time login link that you can use to gain access to your account and set your password. For more about setting your password and ensuring that it is of a satisfactory strength, please see Logging in, requesting a new password.