hey Mitch! Welcome to the forums!
Pre-shading is where you airbrush the panel line highlights on BEFORE you put the main colors on. I don't do it myself but it does work. From what I understand, when you spray the main body color you do it in several light coats as to let the pre-shading still stand out a bit. If there are multiple colors in the camo scheme (i.e. spitfire with the earth brown and dark green) once the top coat is on you mask and preshade the panel lines again where the other color is going over.
Personally I prefer a wash or nothing at all to preshading... just personaly preference... washes and pre-shading both look equally as good!
As far as airbrushes, you can't go wrong moving to a double action.. there are a lot of good ones out there... if you go back on this page and maybe a few pages back there are a lot of posts about good airbrushes. You can see whcih ones sound good to ya...
What kind of modeling do you do? I am assuming aircraft because of the preshading question.. anything else? That can affect which airbrush choice is right for ya too...