Best Ways to learn Keyword Research

I know by looking at the title... some would say to just go out there and learn.. but I want to build a general framework in approaching keyword research.

What guidelines should I be following and I am familiar with the low hanging fruit concept but what else is there that I should keep in mind.

Also I am trying my hand at PPC marketing...

so should I go paid or non paid for some of these services ??

And I am also trying my hand at PHP so I am sure this should help as well... ??

Anyone ??