Given you're using RHEL, SELinux is likely already installed. To confirm:
$ rpm -aq | grep -i selinux
You'll see packages similar to the above. This is from F19, but should apply to your situation as well. You can also confirm using the
SELinux status: enabled
SELinuxfs mount: /selinux
Current mode: enforcing
Mode from config file: enforcing
Policy version: 24
Policy from config file: targeted
There's a warning/note within this document if you're adding SELinux to a system that was initially setup without it.
If the system was initially installed without SELinux, particularly
the selinux-policy package, which was added to the system later, one
additional step is necessary to enable SELinux. To make sure SELinux
is initialized during system startup, the dracut utility has to be run
to put SELinux awareness into the initramfs file system. Failing to do
so causes SELinux not to start during system startup.
When dealing with RHEL it's advisable to seek out the excellent manuals that Red Hat maintains for any RHEL release. SELinux is well documented and everything you need to know is covered there.