The
Shadowrun Quick Start Rules are a free PDF that might give you something to just take a peek at, to get started. They include a mini-adventure (good old
Food Fight) that offers a glimpse into some of Shadowrun's setting, too.
For novels, I'd check local libraries if possible, but if you really want to dive in head-first I think eBay is still the way to get the best bang for your buck; you can do a search for "shadowrun lot" on there and sometimes get a real treasure chest. Since the quality on some of the SR novels is kind of hit-or-miss (often due to stylistic choices made by any given author, or the overall power level they choose to focus on), well, it's often safer to cast a wide net, and try to haul in as many of them as you can, as cheaply as you can.
The only book you'll unequivocally
need is, as mentioned, the "core" SR4A (Shadowrun 4th, Anniversary edition) book. Anything past that is optional, and largely dependent upon what's going to be popular in your particular campaign.
Street Magic for mages and adepts,
Augmentation if folks are already drooling about being awesome cybernetic killers,
Arsenal for a plethora of guns, armor, and vehicles...etc, etc. For setting information
Seattle 2071 is the way to go for a single-city campaign (in the traditional SR default setting),
Sixth World Almanac has more of an update on more of the world, and an expanded timeline that might help you get up to speed. Other sourcebooks beyond those couple, and their utility, are really going to be very campaign dependent. Hopefully some of your friends -- the ones volunteering you to run a game that's brand new to you -- will have a few rulebooks/sourcebooks of their own that they can make available to you.
One fairly recent product that I think might be a big help, though, is the
Runner's Toolkit. It's a fancy-schmancy boxed set that comes with an adventure, compiled gear tables (from several of the books I already mentioned), some handy quick reference sheets (almost like mini, character-skill-specific, GM screens for various players), and a plethora of other stuff that's all aimed squarely at new GMs, to make their lives a little easier. I don't suggest it to everyone, but for someone that's new to the game, but who's made up their mind to give it a shot, I think it could be pretty handy for you.
Not, buying stuff out of the way? Start like you would any other campaign, in terms of trying to get things rolling. Talk to your players and get an idea of what they want to play, cross-reference if you can to parts of the book (if any are available to you just yet) that look do-able to
you, and start to communicate and coordinate with your players. Shadowrun's a big game world, with a lot of different themes and flavors and power levels available to it. One of the first jobs of any campaign is to get everyone "on the same page" and set realistic expectations for what will and won't fly in
your game. If one dude wants to be a barely-getting-by street ganger that's trying to get a little respect as a Shadowrunner, and another dude wants to be Iron Man or Wolverine, you'll be in for a long, frustrating, campaign, trying to juggle them.