With the help of this guide you gonna be able to reset your computer, completely wipe it out, photos, files, apps, everything will be erased forever, but first, why would you ever do it??
There may be times that you wish to start over, from scratch. It may be that your disk is so chaotic with applications or you just wanna install new stuff that you really need, or it may be that you are selling the machine – or passing it on – and you want to revert the Mac to factory settings. By doing this, you ensure that none of your personal data remains and the recipient will get a it with an operating system that is as fresh as when it first left the factory.
It will look like this:
6. Enter Disk Utility to erase the hard drive. Click on Disk Utility > Continue > Select the main volume > Click Unmount and then Erase. Quit Disk Utility.
7. Click Reinstall OS X and Continue. Follow the instructions to reinstall Mac OS X (or make the new owner do it! :P)
You are all set!