Some common PayPal problems:
"You have already signed up with this account information"
This message is from PayPal and usually indicates that you're creating a new account with an email address that is already signed up with PayPal.
You should re-check your information and make sure you're using the "Pay with my PayPal account" form and not "Don't have a PayPal account".
Can't remember your password
You can request a password reset with the "Forgot email or password?" link on the PayPal page below the Log In button.