Is this a new installation or has it been running for a while? What user are you trying to log in as? MySQL and CentOS don't (typically) share the same list of users. Have you tried the user root with no password?
You can always reset your root password. You'll have to shut down mysqld in order to do this, so make sure no one's using it...
Stop the mysql service.
service mysqld stop
ps aux | grep mysql
If there are any mysqld processes still running, run service mysqld stop again
Start MySQL with the following option. This will let you log in as root with no password.
Open another terminal and run
mysql to enter the MySQL shell.
mysql> UPDATE mysql.user SET Password=PASSWORD('MyNewPass') WHERE User='root';
mysql> FLUSH PRIVILEGES;
Your password is now changed.
quit to disconnect mysql from mysqld and then press ctrl+c in the terminal in which you ran
mysqld --skip-grant-tables. Run
service mysqld start to get things going again and you should be able to log in to phpmyadmin as root.