There's a great set of introductory tutorials on Making Games in ActionScript 3 using FlashDevelop at prototyprally. That's where I got started.
And of course there's Flixel, the free AS3 game engine, and a great Basic Game Tutorial I'd recommend for anyone looking to get started with that.
I just posted a more detailed guide on my blog if you want more info on it:
Where to Start with AS3, FlashDevelop and Flixel
Hope it helps. :)