1. If necessary download G-Parted and burn a CD/DVD. You may need an external USB drive to do the burning. Google the instructions as necessary or use a PC with Windows to do the same. Skip this step and number 4 if you can and use 2,3 and 5. Note if you use 2, 3 and 5 then the USB thumb drive in already in the machine.
2. If you can access the App store search under Purchases or Updates and download Yosemite and make an OS X usb install thumb drive. Google the instructions or download the appropriate DiskMaker (free) app to make the OS X installable thumb drive.
3. Put the USB thumb drive in the mini, hold down the option key to get to the Mac version of the BIOS and verify that the USB thumb drive is recognized as a valid install device. It has to be recognized in the bios and it MAYBE "yellow" in color and go to step 5
4. Connect the USB burner to the mini and restart while inserting the G-Parted disk you burned earlier. GParted has an HFS+ function, use that to format the hard drive. Note: I do not recall the partition format but it should be hfs journaled+ or something like that but this link should be helpful. https://support.apple.com...cale=en_US
5. After that insert the USB thumb drive, restart, hold down the Option key to get to the Mac Bios, select the thumb drive and press the enter key. When you reach the splash screen there should be a Disk Utility selection to automatically format the hard drive.After formated Quit Disk Utility and select Yosemite to install. It should go automatically from there.
This is how I recall the process but remember Google is your friend, of course you have to have another machine to get to Google if there is a problem.
I am not on this site often so if you make a reply or things do not go well it could be days or weeks before I am back here. Remember Google is your friend....