WELCOME TO THE DENTED HELMET!
Your helmet looks pretty sweet bro! What did you make it out of?
As far as the molding goes, I would definitely use a thixotropic silicone. These types of silicones thicken naturally so they can be brushed easily. They also fill undercuts very nicely. I generally use smooth-on for high priority projects. Their rebound 25 silicone is one of my favorites because it is a 1:1 mix and is a brushable formula. For the mother mold, you can do a lot of different things. You can use fiberglass, trowelable plastic, plaster bandages, etc. It really depends what you have access to. IMO, trowelable plastics such as plasti-paste are the easiest to use for beginners. You spread them on like icing, and they cure to become VERY hard. You could also use fiberglass resin and cloth to make a very nice mother mold. Just be sure to wear a respirator and work in WELL ventilated places. Fiberglass isn't good for you. Plaster and plaster/medical bandages work well, but they aren't my favorite.
Well, there is A LOT more to it, but I think this will get you started. Don't hesitate to ask questions. There are TONS of experts here.
Kody