Both thinning and pressure of air is a very personal thing.
Essentially you need the paint to go through the airbrush without spattering, thin it some more if it is OR up the pressure a touch, both will result in better atomisation, if you thin it too much it will spider across the job, so turn the pressure down. I'd suggest to get the feel of the airbrush get some thick paper or board and practice on that.
Play about with using it and see what you get, try and make an area of block colour so it's even. Try writing your name. Then draw lines with it so you have a noughts and crosses style gird, now you've got the grid try and put a dot on each intersecting line. Once you're happy with all that try it again on a cut in half lemonade bottle ( much cheaper to practice on than body shells ).
Because paper is porous though it's slightly forgiving, you'll notice you need to have more control on the plastic surface as obviously the paint can't soak in so it's very easy to go too wet.
Hope that all makes sense, post up if you're stuck on anything, i'm sure one of can help you out. Good luck
