What helmet do you have? is it paint already? for full painting you need to use primer, paint and finish coat. all this 3 steps most be based on that same materials (acrilic for example). If you are really need help - post pictures here and read manuals.
ok. the most easy way - sandpaper(240+\-), acrylic primer (ballon), acrylic paint(ballon+brush), acrylic varnish.
other paint secrets you can see on youtube channel of Evil Ted Smith or Stan Winston.
You need to practice for understanding.
We will wait progress photos.