First off, you need Macromedia Flash. That's the program used to create flash games. This program costs a significant amount of money (around $800), but you could always download a torrent of it.
Then there's the matter of crafting your game. If you don't know much about action script coding and such, then I suggest you don't bother. I started not knowing anything, a year later, I still know pretty much nothing. Of course, there're plenty of tutorial sites, so if you really want to, you can give them a look. It's really just a matter of searching "Flash tutorials" on Google.
IF you do plan on getting/downloading the program, I suggest you go for Flash professional 8. I have that one, and according to a couple of people I know, it's the easiest to follow (not saying that it's at all easy, though).
Keep in mind, Flash coding is like a language. You don't learn whole languages in a couple of months. It takes many years to learn.