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What version of Mac OS X are you using ?
Restart your Mac and immediately hold down Command and R keys at the time for 20 seconds and this will boot into Recovery.
Or restart your Mac and immediately hold down Command, Option and R keys at the same time for 20 seconds and this will boot into Internet recovery. This will only work for Macs that came with and has 10.7 or later version of OS X.
I will send you instructions in here, so you can do them at your convenience.
If the above is not working and if you have a working Mac there then you can create an OS X bootable USB drive using this application Disk Maker --> http://diskmakerx.com/
Note : You need to download a copy of OS X from App Store and also need an 8GB+ USB drive to create the bootable drive.
After creating the USB OS X drive, put it on the Mac that you want to erase, then restart and immediately hold down Option key on your keyboard for 10-20 seconds and this will bring startup manager. From startup manager, select the USB drive and press return. This will boot into OS X recovery (from the USB drive) and then you can erase the hard drive and install fresh OS X.