Helio, first off, make sure your helmet's crashworthiness won't be compromised by certain finishes. Some can break down the materials used in some helmets. Usually, most helmets with the latest Snell and DOT ratings should be able to handle it. (but I am NOT an expert!)
If all that's okay, make sure you sand the helmet well, taking off all the shiny stuff, but don't make any flat spots. Then, once you've got a nice dull finish, hit it with a good primer/sealer (or take it to a pro). It's also best to have a pro do the painting (which is what I do). Then after that cures, apply your stickers, then hit it with the clear coat.
You could probably do it all with bomb cans, it all depends on how high-quality you want it to turn out.
Good luck! I hope this translates well to Portugese!