Here is what I did to get comfortable with the air brush:
I went to an art store, got a pad of newsprint paper. Filled the air brush with water colored with food color and started spraying.
The exercise is to draw dots of uniform size and color density. Once that is mastered try drawing lines, both vertical and horizontal. The goal there is to make a uniform line, with no blotches at the beginning or the end.
I used water colored with food coloring because it is very easy to clean up, non toxic, and no harmful fumes.
The next step is to take a few plastic drink bottles, and spray them with rattle can primer. Then put some paint in your air brush and repeat the above steps. This will give you a transition from a flat surface with water to a surface more like a model without actually being a model. At the end of this, you will be ready for using it on a model.
Just my 2 cents of advice......