It's good that you're looking to make a simple game to begin with, but let's get some information so we can help you to get started properly. Obviously you've chosen to use ActionScript3 to make a Flash game; this is a good choice, and one that will allow you to make some money off of your games once you've learned the basics.
1. To begin with, what operating system are you using? If the answer is Windows as I suspect, which version are you working with?
2. Secondly, have you chosen which tools (Adobe Flash, Flex Builder, FlashDevelop, HaXe, etc.) you will use to create your games... which one are you using? Have you got it installed on your computer and ready to use, or do you need help setting up your tools?
3. Next, do you have any experience yet, or are you starting from scratch? If you haven't learned a few basic things such as loading and displaying a graphic on the screen, you'll probably need to go through a few quick tutorials on that before we look at starting on Tic Tac Toe (naughts and crosses for anyone unfamiliar with that name). Note that we're not going to just give you the source code to a tic-tac-toe game, and if we did it wouldn't really help you to learn -- but we can help you out with tutorials, resources, advice and trouble-shooting to help you write your very own!
I'm not going to offer any further advice or links yet, as it may or may not be relevant based on your answers to my questions above -- help us to help you, and let us know your answers to those things so we can offer further information!
: Added in additional content after moving topic from 'Game Design' to 'For Beginners'.