Hi moralna, I ran across your thread in the unanswered forum section, and though I personaly do not currently airbrush anything, I do have a Brother-in-law who has been a professional airbrusher for over 30 yrs.
Advice to you if you are serious about persuing this, is to look for classes in your area on airbrushing and transfer that knowledge to decorating for your cakes. Or you could purchase videos and learn at your leisure and again apply your knowledge to cake decorating. Here is a link for some videos just to give you an idea of what is available. Once you've decided which direction you are going, you can begin to research equipment used for airbrushing cakes. Have fun and Happy baking. 
Gloria I
http://www.dixieart.com/AirbrActVideos.html
You can airbrush letters using a stencil or freehand, but you really need to practice a bit before airbrushing a real cake. I'm by no means an expert, but I started out by practicing on newsprint. You really have to get a feel of the airbrush, how to hold it, how to apply the pressure. At first, I didn't even know where the color spray would end up. So, practice and protect the area from overspray.
As morninglories said, videos or classes can help.
Good Luck!!!!
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