How do I fix this?

Well, first, you want to get a hammer. Make sure you're comfortable with the weight, you're gonna be doing a lot of swinging.

Next, you'll want this hunk of metal called a dolly.

Now, if you're someone without experience in a body shop, I'd recommend searching "Dolly dent removal methods", watch some tutorials.

If you're familiar already, you know what comes next. Decide if you're going on or off, and beat the shit out of it until the dents come out.