If you already know how to do some basic programming, this shouldn't be too hard. The way I've always done turn-based stuff is by using a boolean to represent whose turn it is, and using if-statements to lock out incorrect moves. However, I'm not very experienced with ActionScript, so there might be a much better way to do it that I'm missing.
If you don't have much/any experience with ActionScript or a similar programming language, you might want to google a tutorial for this. There are many good resources available.