If you installed phpmyadmin using apt-get then your mySQL Server should be installed with the user root and no password.
If you installed manually by downloading the phpmyadmin files and installed on a web server you will need to install mySQL.
apt-get install mysql-server
this will start your mySQL Server. Your connection details will be
User : root
Password : <blank>
Port : 3306
You can create a database by running
mysqladmin create <dbname>
Once you have done this your config should look like this
$dbhost = "localhost";
$dbuser = "root";
$dbpass = "";
$dbname = "<dbname from above>";
$display_errors = false;
I highly recommend you set a root password on mySQL however and never run queries as root unless creating new databases or adding users.