If you really do need to do that, then the broad, outline procedure is something like this:
1. find a USB stick, or USB disk with capacity of 8GB upwards that you are prepared to completely wipe
2. Re-format that to be a installation disk - this will involve downloading a 5-6GB installation file from Apple.
3. Restart the Mac and tell it to do a new install from the USB drive .
I can walk you through the procedure in detail, but it would be good to know why you think this rather drastic procedure is necessary.
Do you, for example have a Time Machine backup?
Have you tried some of the disk utilities to repair the drive?
Also, it would be good to know what kind of Mac - Mac OS version number - amount of disk space and so on. This is probably not essential, but might flag up some issues
Good luck