If you already have a Games With Friends account and you have NOT yet set a password you will need to set one:
Alternatively, if you have the original device your account was created on you may use the following instructions:
Launch Words With Friends on the device you created the account (iPhone, iPod touch, iPad, Android, Windows Phone).
Tap on the "Menu" button.
Tap on the "Settings" button.
Set your new password.
Launch Words With Friends and login with your newly set password.