Just to expand on what TinkerGnome was talking about with combat. The type of campaign that SR is is similar to a D&D city only campaign. If you've only been playing D&D a year it is quite possible you have never played a city only campaign, just dungeon and wilderness campaigns where you go into a small town or a city to restock. Most dungeon crawls and wilderness monster bashes are fundementally different than a city campaign in that you are almost certain to in a city come across unfriendly or even aggressive NPCs that are hugely more powerful than you, and therefore extremely dangerous. Because it can be hard to judge how tough an NPC can be. So the
default plan should be something along the lines of stealthy avoidance.
Then there are the civilities of the city. Not walking around with obvious weapons. I guarantee right now you'll have someone somewhere along the way declare they are taking the bus while carrying an assualt rifle/sinper rifle/Panther Assault Cannon and a bandoleer full of grenades. It'll take a while to get the mental picture of what is going on. That the metroplex ain't no dungeon crawl.
For you as the GM the toughest part is likely that your characters can, and likely will go all over the place and do all sorts of things that you never expected. Reading and understanding the setting is your best tool here. It will allow you to fill in some details as they go. The second thing is having a pool of NPCs, and being able to also draw up the core of an NPC on the fly. Make sure to write down the name of the NPC and his general abilities incase this ends up being a recuring NPC that they have to go back to later.
Anyway, disposable characters are an excellent idea. Those are the best for finding out an itchy trigger-finger is the route to an early grave.
If you go over to the Community subforum of the main Shadowrun forum you'll find a few character generators that should help a lot when the players do get around to making their own characters. They could even play around with them right now just to get a feel for what is out there.
Something that I'll also toss in that I find helps a bit is to take a dip in the pop culture pool and watch a few movies to get them in the SR mood. I don't just mean sci-fi, although Bladerunner is an obvious choice for atmosphere and one damn fine movie to boot. Just glancing through my DVD shelves you could check out Boondock Saints, Leon:The Professional, or Way Of The Gun. If you like things a little on the lighterside maybe Wasabi, Snatch, Lock Stock & 2 Smoking Barrels. The Transporter 1 & 2 is a great rigger/driver example of SR type action and the over-the-top Hollywood style you could have. Then there is the killer superspy Borne Identity movies, and in that vein the grittier Ronin. Heat is great for that criminal team feeling. There are a million and one heist movies. I notice The Italian Job on my shelf, but that's mostly for my wife. ;P If you are going out to the theaters anyway sometime soon I recommend you check out The Matador. I think a great example of a hitman and his fixxer. There are lots more, and numerous threads in the main forum on this. Just do a search on "movie" and set the time to a year back and you'll get lots of threads with lots of ideas for this.
If you want to understand the roots of where SR grew from and the source of a lot of these goofy ideas in then you can buy Neuromancer. Which is what FASA originally ripped a lot of stuff off of back in the 80's for SR1. A fairly quick read. Notice the Julius Deane character, the prototypical gear fixer.
Good luck and have fun!
EDITOne last piece of advice. Keep it simple! You don't have to have them meet every iconic NPC on their first run, or have them make the Big Haul or play the pivotal role in world changing events. Just keep them some Joes doing a job. It avoids that whirlwind tour of the stars cheezy feel. Just play the game, the good times will come out of that. It certainly will be a lot easier on you to GM.