AS1 would be futile as not even Flash Lite makes use of it any more. AS2 is very similar but still being used. If you’ve got a good understanding of object oriented programming, I’d suggest going directly to AS3. However, if you’ve little programming experience, start with AS2 as it is far more forgiving and easier to break into though it does have issues (mainly scope issues which AS3 resolves). I would suggest aiming for proficiency in AS3 if you plan to do any serious Flash development professionally.
As for where to learn, there are a bevy of resources on the web. You shouldn’t have much trouble finding forums and tutorials to meet your needs. The Adobe documentation on AS2/3 is pretty decent and worth referring to when it comes to technical information.