Get an actionscript book or something.

Seriously, the best way to learn any scripting language is to start with an ambitious project (not too ambitious - one you can achieve.) Then, when you need to do something, look it up specifically. The best resource in my opinion is the Flash documentation, especially the Actionscript Dictionary.
I am relatively experienced with AS, having built my own online Operating System in it, and I have used Flash in several large multimedia projects for the Australian government. I would be happy to answer any queries you may have.
