Double check your email address and password to make sure that all of the information is entered correctly. If your are still having trouble logging into your account, you can request a temporary user name and password.
Also be sure that you verified your email address. When you first sign up, you receive a verification email with a link in it. You have to click the link to finish the sign up process. If you didn't do this, you won't be able to log in. If you can not find the email, please check your spam or Gmail Promotions folder for an email with the subject "Activate your Account." There is more information about finding missing emails here.