I've been a GMing games since the days of D&D 3.0, and I have to say that GMing PFS scenarios is pretty easy to jump into doing. You can get by quite readily without knowing much about the setting. If you would like to get a good introduction to the campaign at the same time as you start GMing, I have a few comments.
First, there are some scenarios that are more typical Pathfinder Society (the in-game organization, not the organized play organization). Starting with low level scenarios will make your job much easier.
- 5: Mists of Mwangi and 2-11: Penumbral Accords (set in the same museum)
- 29, 30: Devil We Know series
- 35: Voice in the void (in the basement of the museum of 5 and 2-11)
- 37: Beggar's Pearl
- 39: Citadel of Flame
These are all fun and stereotypical Pathfinder adventures, and they will give some some information about Absolom. At the end of July, Paizo will start releasing a new series of scenarios intended to introduce players to PFS and the factions - The First Steps series.
Second is that preparing to run PFS is not as difficult as preparing for your own custom game. My first step is to read the scenario and highlight important things via highlighter (if printed) or in the PDF itself. Important things include Skill check DC's, faction mission details, and how NPC's act or react. Once I've done a good read-through, I then focus in on the combats likely to happen. Each NPC or monster that has a stat block includes strategies for what they will do in combat or prior to combat. If there are abilities or spells I don't understand, I look those up. If you do not know what levels of characters you have, you will have to study 2 or 3 sub-tiers in preparation. That's the on part that can be difficult to do since you have to remember some differences on occasion.
For paperwork and reporting, I always print one set of faction missions, one copy of any player handouts, and eight copies of the chronicle sheet for the scenario. Once you've filled out a few scenario sheets, you'll get the hang of it, but don't be afraid to compare what you've filled out with an already completed sheet of your own or someone else's.
The most important thing is to have fun though, don't worry too much about knowing every single rule or every single class/spell/feat/etc. Almost every table has someone who knows a lot of the rules, and they can help you out on things you don't know. To keep the game from slowing down, sometimes you have to just make a ruling with what makes sense and then take note to read up on it afterwards.