How do I factory reset my Mid-2013 MacBook Air?

Hi, I have a MacBook Air, mid 2013, OS X 10.9.4. I need to reset my computer to open up space on my hard drive. I do a lot of work on iMovie and have tons of "videos" on my hard drive that I can't seem to find so I thought it would be best to just reset the thing. Now, to the question.

How do I factory reset my Mac?