No offense Calj but that sounds like a recipe for disaster. He has NO welding skills. Cutting a hole in the bottom of the tank means petrol sitting on the weld, any pinprick hole will mean a leak. Cutting a hole ABOVE the dent is a much better idea IF you really want to cut a hole at all.
Take it to someone who can weld, get them to weld a rod or 2 onto the dent and pull the dent out with some sort of slide hammer is my advice.