Hi, welcome to LQ forums.
To install third party repositories take a look at http://wiki.centos.org/AdditionalResources/Repositories
. But first please learn a little about repositories so you do not mix several repositories at once! Once you set desired repository(ies) you will use "yum" command to install update like the normal repositories.
CentOS is practically free version of RHEL, and 99,999% binary compatible with RHEL, so if you have not payed for RHEL (support), and you have no intention to do so in the future, please reinstall your system with CentOS. Advantages are continuous updates without need to pay anything and the fact that 90% of people here will tell you to call Red Hat support every time you you have a problem that does not cover normal operation and learning process.