Set the root's password:
sudo passwd root
Enter the password twice. Now you can use "su" to enter root account.
But how to disable root's account? The answer is "passwd". In your normal account, type "sudo passwd -l root". Here, the argument "-l" means "lock". It locks the root account. However, you can unlock the root account by typing "sudo passwd -u root" if you have set the root's password. Here, the "-u" means "unlock".