Purchase the supplies you'll need. You can find them at art stores or online. Airbrush supplies are readily available, and you can find deals on complete beginner kits. This is typically the best way to go if you are just getting started. A kit will contain all you need, usually at a better price than purchasing every item individually. A lot of kits even include a getting-started DVD.
Find a website community of airbrush artists willing to share their expertise. There are many such websites on the Internet (Links to websites are provided in Resources). Many of these websites offer free articles and videos to help you learn, as well as an active message board community willing to answer questions and share airbrushing information with you. One or two of these websites can be a helpful guide as you learn how to airbrush. There are also several airbrush magazines on the market. Consider subscribing to one of them. Another excellent learning resource for airbrushing is Youtube. Watching an airbrush artist can be beneficial to your learning process.
Get started with something as simple as a lightning bolt. Choose your work surface. A blank painting canvas or even an old t-shirt will do the trick. If you don't have the skill to draw anything, purchase a few stencils from an art shop and use those to get you started. The best way to learn to airbrush is to actually do it. Don't worry about messing up a canvas or an old shirt.
Load your airbrush gun with white paint and hold it five or six inches from your work surface. Start at the top and spray a long zig-zag line for the center of your lightning bolt. Apply the paint a couple of times to thicken it up, then add a couple of smaller zig-zag lines coming off the main one. Clean your airbrush gun and load light blue paint in it, which you can apply lightly to your lightning bolts to give them depth and shape. This is a simple project that will get you used to handling your airbrush gun.
Practice frequently. As you continue to go through your learning materials, always find something to airbrush on. Try making subjects up on your own, and copying from pictures. The more you practice, the better you will become at handling your airbrush and being able to paint what you see.