I would suggest reading everything you can get your hands on. Kalmbach publishes 'How to Use an Airbrush", available in the bookstore, and there are numerous articles in Fine Scale and other modeling magazines that show specific techniques and applications.
Another thing that really helps is practice. Rather than spend money on model paints and plastic, you can start with india ink and posterboard until you get a good feel for your AB. I've used an airbrush for years, but I always have some scrap black (for light colors) and white boards handy to check the spray pattern or get a feel for a new type of paint, tip size, etc.
Hope this helps, and have fun!
Mike