So I have done this before on my iMac but I am trying to help my sister do it as she is having several issues with her Macbook Pro 2011. What you do is you hold Cmd + R until the Apple Logo appears. Then click Disk Utility and then on the left, click the Macintosh HD Volume and click Erase, name the volume and the format OS X Extended (Journaled), and finally hit Erase. Wait a while for it to erase and then back out of Disk Utility and then click on Reinstall OS X and again wait a while. The part I'm having trouble with is when my sister goes into Disk Utility to erase the Macintosh HD volume, its greyed out and I have no idea why or how to fix it. Help would be much appreciated!