Much has changed from as2. You should probably do a little introductory reading on that before diving in. Check the resources sticky thread on top of the forum for links.

In a nutshell, the "on" handlers have been removed in favor of addEventListener. Also all properties that used to be preceded by an underscore ("_") now do not have the underscore. Also also, Key.isDown is no longer native, though senocular has re-implemented it in a package that I've found useful. http://senocular.com/flash/actionscript.php