remove unnecessary contents
 

How will I check the unnecessary contents of my system and remove them ??

what are unnecessary contents ?

It can be dangerous, especially with RHEL. They have a great many dependencies, many of which may not make sense to us.

By unnecessary contents, do you mean programs, files that you no longer use, or other things?
You can get a listing of all installed programs by doing something like rpm -qa > rpms.txt. That will list all rpms.

Then, doing yum remove <program name> will show you what else might be removed. For example, you might want to remove wireless-tools and find that it will remove most of KDE. So, you want to be careful, and never do yum -y remove till you see what else it might take. (For example, as mentioned above, if you have various KDE programs installed, at one point, on Fedora at least, doing yum remove wireless-tools would take out most of the KDE programs.)

I'm not sure if I've answered your question correctly, if I've misunderstood, my apologies.

I have only the OS installed and some s/ws and its taking more than 25GBs,So how will I know all 25 GBs are necessary ,thats why I asked the question ...

Decide what you want to use, you can uninstall games, servers (apache, ftp, ssh...) docs.....
However, if space is an issue, why not install DSL, puppy, debian netinstall....