Have you already perused
this thread? Well briefly, a single action airbrush works by pulling down on the trigger. That controls the flow of air across the needle, siphoning paint from the jar or color cup or whatever. Paint volume is controlled in a static fashion, by setting the depth of the needle with a screw. Double action airbrushes also control the airflow by pulling down on the trigger, but the paintflow is controlled by pulling
back on the trigger. This allows you to adjust the volume of paint on the fly, as it were.
There are any number of books and magazines that will give you exercises in how to use both kinds of airbrush. But the only real way to get good at it is- you guessed it- practice, practice, practice.
I speak of the pompatous of plastic.