This is a question a lot of people ask buddy. If you want to make a game with flash, first ask yourself: have I ever made a game before? If so, what language and/or program did I use? Was it any good?
If you can answer all three of these questions, making a game in flash shouldn't be all that difficult. All you need are the following three things: an idea, flash, and skills of a programmer/artist. With these three things in tow, making a game is only a matter of learning how to use flash.
Go ahead and download CS4. You'll have about 30 days to make something, this should be enough time to make or break you. Once you've got the program, look up AS: Main and check out the Tutorial Collection. Go through as many as you can within the first 14 days of your trial. Once that's done, spend the next half of your trial period, day 15 - 30, coming up with a solid introductory idea: pong, tetris, connect the dots, Escape the Room, etc. Make sure the game is simple and requires relatively little work to complete. During this time, you're going to use what you learned from NG to build the game in flash. As you go along, you'll need to refer to the previously mentioned resources to get you out of any issues or stand-stills. By day 25 - 30 you should have the game completed.
Once the game is completed, simply upload the file to NGup or whatever upload site is still around and make a new thread about it. Have us go through it and suggest any changes you can make, or any bugs you need to work out. If the process is mostly positive, you should be well on your way. If you find that the criticism is too harsh and your feelings get hurt, then move on to the next thing because flash isn't going to be for you.
If after everything is said and done you still want to make flash games, all you need to do is purchase a License of flash and get a couple of books. Commit a lot more time to learning ActionScript and sharpening your artwork (if you have any skills in that area). Continue to familiarize yourself with the environment. After about 6 months you should have enough resources to get your through your first real game. If you stick around here, you should also have built up enough connections within the Flash forum to get any sort of help you may need.
Please note that the above is an over simplification of the process. It'll take a lot more work than conveyed in this post but patience should get you through it. If in the end you just don't have what it takes, don't fret. There are programs out there that will let you make games with little to no skill in any area of video game creation. One example that comes to mind is RPG Maker 2003.
Good luck :D