If it is water based acrylic, you can thin it with Fantastik cleaner, if it even needs it. Most airbrush paint is thinned enough for an airbrush, but if it sprays droplets, add a little thinner.
First coat, spray very lightly. Especially if you have masked any areas. Spray a light dusting along the edge of your masks and let it dry. It should be barely visible. Do this a couple times to seal the edge of your masks to prevent paint bleed.
After you give the entire body a couple light coats, go back with normal coats, letting it dry a few minutes between coats.
Start with your dark colors first. Make sure to hit any light spots before switching colors.
Back the paint with white for light colors, or black for dark colors.