How do you repair a dent in the wall?

It really comes down to how solid the dented section is. If it doesn't flex when pushed on you might like to just skim coat it all until flush with Gyprock CSR 2kg Rapid Patch Filler. It would be best to do so gradually with several coats instead of one thick application.

Another option is to cut out the affected section and install a plaster repair panel. This could be an easier repair option and would require less filler added to the surface of the wall.

If you don't replace the bent area with a new plaster sheet you will need to apply filler to the entire area to try and return the wall to a flush surface. Even after doing so, you have only filled over the top of a damaged section of wall and there is a reasonable chance that the filler will crack as the sheeting is not sound.

You should find this step-by-step guide useful: How to fix a hole in a plaster wall. - MitchellMc