For my first airbrush I got a
Badger 250 so I'm partial to that model. You can buy propellant, or pick up a compressor if you want. If you have a junker figure or model kit handy, practice on it first so you can get used to the airbrush. Better brushes have interchangeable tips to produce different effects.
For the toy itself, I recommend taking it apart one limb at a time, if you're comfortable with that; it makes for a better end result. I use double sided tape to keep the screws separate and in order. Get yourself a couple
locking forceps, you can lock them in screw holes on the pieces; it makes working with wet pieces a lot easier. For big parts, or things that are going to be one color, airbrushing or using spray paint is best, but you can use a brush if you want (most of mine are brush-painted). Use tape to mask off what you don't want painted. For details or small areas I use a brush. Go slow, and watch your fingers. I've ruined more models with fingerprints than I care to admit to. Let 'em dry for a couple hours (overnight would be better) before reassembling.
If I haven't scared you off so far, let me know.