Still learning meself, but air pressure and paint thinning are two major hurdles to learn.
For general overall painting of large surfaces I use 20-25 psi. with the tip 6-8 inches from the surface I'm painting.
For camo borders I knock that down to 8-10 psi, and move the tip to about 1/2 inch from the model. My airbrush also has a variable needle adjustment incorperated into the trigger so when doing fine spraying (up close) I don't want a bunch of paint coming out with the air mixture. Expiriment on scrape parts.
Paint thinning is vital. Pre mixed airbrush paints are available in some colors (mostly metalizers), I try to achieve the consistancy of milk when thinning. Use a good thinner specificly for airbrushes for thinning your paints. I use cheaper thinner for the cleaning of the airbrush.
Air source is important. Canned air is expensive, and not much good for large areas as they tend to freeze up with extended use. A compressor is nice but if bucks are a consideration, I heard of people that use a tire innertube, with a small inline regulator.
Hope this helps some.