Something to hold it still - like a vice. A blow torch to heat the metal. And a hammer to beat it back into shape when hot.

Or if you don't have the first two, a hard flat surface like a driveway and use your body weight to hold the bike flat and down, and a hammer to try and force the metal bar back.

It's basically the use of the same force, that bent it, in reverse. So if someone sat on it, think of how force you need to apply. It's not a small amount.