Just to add on to what dirtdives said, here is a pretty popular practice: put one rather light layer of resin (either epoxy or ployester/fiberglass) on to the outside of the helmet. Once that has cured, do one or two light layers on the inside of the helmet. Now you will have a rigid, but fragile, shell. At this point you can add a couple layers of fiberglass to the inside to give it strength. Not you will want to start sanding the outside where the details must be rounded, and use body filler if necessary. Now this subject has been beaten to death, but I'll say it once more here, DO NOT USE MUCH BODY FILLER. If you put it on in layers more than 1-2mm thick, you are using too much, and will spend hours sanding it back down. Also keep in mind that not all of your helmet needs to be hit with body filler, only the round, smooth parts.
Due to your questions on rather basic stuff, I'd strongly recommend doing a little more reading through build threads and tutorial threads, as it will help you out greatly to have thorough knowledge of other members techniques before you start!