How to format my compuer

It requires various steps

You have to first think of backing up your data, then arrange for OS cds

Make partitions using FDISK
Insert CD and install OS

This is overall you have to do

First ask yourself if formatting is the only answer to you problem as to do so is total and irreversible.
Next do copies (on disk) of all the work you do not want to loose and put in a safe place
Ensure that you have a startup disc which holds all your operating system info.
Boot your PC with starup disk and when in "dos" format drives
Repartition (in wanted) and reload full OS