Step 1: Getting the program
The best place to get it is from Adobe.
http://www.adobe.com/products/flash
If you find it too expensive then you can get a free one at http://www.soft32.com, it is not as good as Adobe but it gets the job done.
You can also get a student edition of flash at http://www.creationengine.com, it has all the same features of flash but does not let you make any cash.
You can always give Ebay a try to get a second hand version of flash.
Step 2: Learning Flash
Now that you have the program you can now start using it.
I would suggest using AS: Main for programming expertise.
Glad to help -Johnny
Some of these ideas (of where to get flash) where taken from the flash "The Tutorial Collab '06". Here is the link:
Check out the tutorial.