Hi and welcome to LinuxQuestions.org.
In RHEL you usually install software from Red Hat Network for which you need an active subscription.
With the following command you are able to register your system:
Then you will be able to install software with yum:
# yum install packagename
If you do not have an Red Hat subscriptions you may have a look at Fedora or CentOS. There you may install packages from public yum repositories.
To add new repositories you have to place .repo files in /etc/yum.repos.d.
A repo file may look like this example:
$ cat /etc/yum.repos.d/rpmforge.repo
# Name: RPMforge RPM Repository for Red Hat Enterprise 4 - dag
# URL: http://rpmforge.net/
name = Red Hat Enterprise $releasever - RPMforge.net - dag
#baseurl = http://apt.sw.be/redhat/el4/en/$basearch/dag
mirrorlist = http://apt.sw.be/redhat/el4/en/mirrors-rpmforge
#mirrorlist = file:///etc/yum.repos.d/mirrors-rpmforge
enabled = 1
protect = 0
gpgkey = file:///etc/pki/rpm-gpg/RPM-GPG-KEY-rpmforge-dag
gpgcheck = 1
Some repositories are packaged as RPM so you can import them with:
# rpm -ivh http://url.to/repo.file.com.RPM
Or you can download them from a website:
# cd /etc/yum.repos.d/ && wget http://url.to./repofile.repo