1. Shut down your system.
2. Hold down power + option to start your computer back up.
3. Let go of power, but keep holding option.
4. You should see two hard drive icons, one that says Macintosh HD and one that says Recovery HD.
5. Select Recovery HD, and click enter.
The rest is up to you, if you want a better description of how to do this more accurately there is a guy by the name of Lou from "UnBoxTherapy" on YouTube that gives a great tutorial on how to do this.
Here is a link to his video:
Hope this all helps!