If you don't have access to your account or forgot the password, you can request a new password by entering your email address here:
http://wordpress.com/wp-login.php?action=lostpassword
Please follow the instructions carefully. You'll receive an email during the reset process, so make sure you read it for important instructions. If you don't receive the email right away, check your spam folders as well because the confirmation email can get caught in them sometimes.
Once you get access to the account you can follow the steps here to delete your site:
http://en.support.wordpress.com/settings/delete-blog/
Please read the warnings and advice before deleting a blog. We can't revert blog deletion for any reason.