I'd suggest Gamemaker (although I never quite had the patience for it) or Blitzbasic : Blitzmax (my favourite : Win, Linux and Mac)or Blitz3d (Windows only and good for 2D).
Blitzmax is a bit more programmy and, some say, complex and not free ($80) but really powerful. A few omissions (no video support or render-to-texture and OGG/wav sound only) but certainly good enough for a platformer.
The community is pretty good for answering technical questions although I had to stop posting as they can be quite 'difficult'.
I'll get back into it at some point but concentrating on AS as the moment.
You may like to check out Novashell Game creation system - available for both Win and MacOS, its open source under a zlib/libpng license. Another open-source game maker that I've been using recently is Platinum Arts Sandbox but that's really for 3D, but is also available for Mac and you may be able to make a 2D platformer on it.
There's 2 websites that would be helpful, http://www.gamasutra.com/ and http://www.gamedev.net/. Both of these have been around over 10 years and have accumulated a lot of information. First I start with reading gamedev's forum and library, and gamasutra's articles.
A game development environment, and game engine is only a small part of game development. One thing that you want to look at are game design documents. Look at the examples and take the time to write one out. This will help you plan the game out before even starting on the game itself. And reread it after you finish it, you'll be glad you did.