You should be able to use your own account but it may wish you have access to sudo (if you don't already have it). If you don't know your root password, or your own, change it with passwd.
sudo passwd root # changes passwd for user "root"
Usual password strength lecture applies. If you can't use sudo (I'm not a user of CentOS so I wouldn't know) then you can use an installation disc to reset it using the rescue
option. I believe you can also rescue the password via grub.
I use webmin myself so I know the changepass file exists. You'll need to make sure you definitely installed it to that directory. If not just peek around, with ls, in every webmin folder you can find on the filesystem until you find it.