As the cabbie told the musician when asked how to get to Carnegie Hall, "Practice, practice, practice." The best way to learn this art is to have a mentor who can guide you along. If you don't have a moedl club with fuigure painters near you, use one of these sites. Take good close up pictures in progress and of completed figures. Listen to the critiques. (Unfortunaltely, some folks only know how to say what excellent work the posted picture is, and you don't learn from unwarranted praise.) Unless you are a born figure genius, it will take a long time to learn.
Consider getting Andrea's CD-ROM on figure painting, an excellent tutorial.
One major way painting figures differs from aircraft is that you will be using regular brushes extensively. If you use an airbrush at all, it will be for large areas, such as applying a base color to a cloak and for priming. You'll also be deciding on your medium of choice, the two main types are artists' oils or acrylics, of which Andrea and Vallejo are the absolute best. Don't even think of using Tamiya, you can't brush it or mix it and expect it to stay usable long enough to even paint a 1/35 face in the base color.
Go to http://www.sentinelminiatures.com and check his hints and tips. He also carries many of the international publications that specialize in figures. Andrea press also has a number of books on various aspects of figure painting, e.g., Romans, Old West, girls, etc.