[CentOS] New to Centos and have question about updating packages

>  Now my question is what are the commands for Centos to fetch ,update
> and install, all the packages installed on the server. I would like to
> use Yum. Any help would be great.

Welcome Darrell.  Here is the summary version:

To install a specific package, such as postfix from a repository that 
you are configured to get packages from use:

yum install postfix

To update to the newest version of postfix that is in your configured 
repositories use:

yum update postfix

To update everything in repositories that are configured on your machine 
use:

yum update

If you are moving between point releases (CentOS 5.1 to CentOS 5.2) use:

yum upgrade

Hope this helps.

Barry