You probably received an email from an organizer about signing up for a volunteering position. When you tried to sign up on the website, it said your email was already taken. However, you have never used the site before, and you do not know how your email can already be taken; and you don't have a password.
The organizer already created an account for you. You need to first set a password to your liking before you can log in by clicking the "Forgot Password?" link below the login form or by going here: