Q: I accidently deleted everything on my macbook air, how do I reinstall it and get it back to normal?

I was trying to wipe my macbook air and put it back to its original factory settings/condition as if I had bought it straight from the store and apparently I deleted a little to much and now when I power it back on the screen flashes a file with a (?) question mark in the middle. How can I reinstal the files I deleted and get my macbook back up and running?