I'd recommend you start with Createx paints, because they are true air brush paints, and will give you less problems starting out.
Thin the paint with their 4011 reducer to the consistency of milk, and you should be fine.
Spray thin coats, heat set/dry between each coat with a hair dryer, and take your time.
Practice air brush strokes on a piece of white paper or cardboard, so you get the feel of the trigger and paint controls.
Clean your brush often, and thoroughly.
Read as much as you can here, using the search feature.
Go online to a place like Coast Air Brush and check out some of their free instructional videos.
Go to Youtube and check out air brush videos.
Have fun! That's an order!!!