I think I deleted my start up disc so theres a flashing question mark when ever I turn my macbook on

I just bought a MacBook Air second hand and when I got it it wasn’t factory reset so I followed a video to factory reset and I ended up deleting some disc and when I turned it back on there was a flashing question mark folder so I looked around on the internet for help and nothing worked because it says I don’t have a start up disk so I think I deleted it. How do I get the disc back?