As far as the rules, there are some resources (like
this one) detailing the changes from 3.5 to Pathfinder. Honestly, though, if you're familiar with 3.0 or 3.5 you're most of the way to understanding it already.
If you're switching from 4e, I'd suggest reading the rules as they come up (races, classes, equipment, skills, and combat before starting the game probably). The full rules are available on
Paizo's website, including a lot of little editions.
As far as the flavor, if you're switching settings, the
Inner Sea Primer is the Dummy's Guide to Golarion. The best free source for setting info is the
Pathfinder Wiki.
If you're planning to start GMing in Golarion, I'd peruse the wiki and decide where you're going to set things, then grab the Inner Sea World Guide and/or whatever book covers the nation you're setting the game in.
Cheers!
Kinak