If someone posts their code online you can almost guarantee they expect to you learn from it & they realize that you may use it. Some want credit for their work: putting their name in a section showing who they are & what they contributed, others want to be contacted to give permission.
If they have posted it & you like the way it works, look at the code & learn from it. I've been able to figure out many a problem just by doing that. Now, I did not say download a swf and decompile it. That's a whole 'nother story.
There's blogs, websites, and forums that you can gleen from. You'll see all sort of things and find yourself saying, "Oh, I can use that in this project." & "That is so incredible, I wonder if I can make that."
As for learning, the best way to start is with a project. If it's too big you can make it in sections.
Another good book to get is Actionscript 3.0 Quick Reference Guide. It's good for beginners and for those who have codes in AS2.
HTH
~Po