If you try using Flash Develop, at least for the first few times, I'd recommend using Flixel or a similar engine. That should help adjust you to the actionscript used in FlashDevelop.
Necro thread is necro. Other than that: flixel is a framework, not a standalone program you can use to create games. For programming flash, your choices are roughly Flash CS5, flashdevelop or FDT, of which only flashdevelop is free ;)