how do I reset my macbook air to factory setting

I just bought my macbook air today, and I hooked up my external hard drive up to it to put all my stuff from my old laptop on it. I decided I would rather just start over and hook up the external hard drive after I set everything up and just pick and choose what I wanted off the hard drive. there is a lot of stuff from my old computer that I don't want on my new one. help!