Since this hit my "Shadowrun Missions" signal, I'm obligated to stop in and comment.
If you go to the Missions site, there are currently a variety of Shadowrun 3rd edition adventures there. (Fourth edition ones will start to be posted in March.)
If you're just getting started with Shadowrun, while the game rules won't be accurate for 4th edition, the first 5 adventures are a pretty good introduction to the Shadowrun world.
SRM00-01 Mission Briefing focuses on "A day in the life" of the Shadowrun world.
SRM00-02 Demolition run focuses on how to do a basic Shadowrun.
SRM00-03 FORCEd RECON focuses on stealth, and how to do things without drawing attention to yourselves.
SRM00-04 Fork in Fates Path focuses on making choices.
SRM00-05 A Dark and Stormy Night focuses on what to do when the run goes wrong.
Once you've read (and possibly played) through those, putting together scenarios should be fairly straightforward.
If you're looking to buy a product, while it's an SR3 book, Mr. Johnson's Little Black Book is a great resource. The first chapter is all about how to GM a run. It goes into great detail on all of the little things that a veteran GM often does intuitively.
You've also found a great resource on Dumpshock. If you have specific rules questions, folks will generally be more than willing to help out with them. (Though, you won't necessarily ever find two people who give you the same answer

)