Q: How can factory reset my I mac

this will tell you how > What to do before selling or giving away your Mac - Apple Support

You did not furnish any necessary info such as your model/year and the original OS version on it. Not having that info, the only suggestion is:

Make a full backup of your files first because you will need to erase your hard drive.

Then boot into internet recovery (Command + Option + R); this will take a while; when you see the Utilities window, use Disk Utility to erase your hard drive; when finished, choose to reinstall OS X (unless yours came with install disks which would involve completely different instructions). That is why it is important to include all information.