how can i do it on centos like how to do the yum install quota thing.
CentOS is Linux and it uses "yum" just like most of the other Linux distributions
so to answer your question, you do exactly what you were told above ...
You may already have the quota module installed and yum will tell you if that is the case.
Once that is done, all you need to do is login to WHM and go to "Initial Quota Setup"
and initialize quotas for all your accounts and your quotas should be up and running!