First time GMing? and you start with Shadowrun? You, sir, have big brass clangers and I congratulate you.
GMing can best be described as the art of making your players work very hard for a win, but ultimately making sure that they do win. If the players don't 'win', then they aren't really having fun, but if it is too easy, they get bored.
First, read the SR4A book from cover to cover at least once. Make notes of things that are not immediately clear and go back through the book to try to find your own answers. Note page numbers. Any questions left unanswered can be asked here. Morale: Know the mechanics.
** SHAMELESS PLUG **
Feel free to download the cheat sheet pdf in my sig. I tried to simplify all of the rules into straight forward tests with modifier lists and it has served me well.
** /SHAMELESS PLUG **
Second, remember that unlike D&D or other fantasy settings, Shadowrun has no method of ressurrection, so be careful about throwing out massive damage you can get a total party kill (TPK) really quick. Likewise, healing in Shadowrun takes time and it's as simple as casting Cure Light Wounds. Even the Heal Spell takes time to make it work. Morale: Don't start at full throttle.
Third, for your first game, keep the background setting simple. Let the players know that yes everything in the books is out there somewhere, but ultimately it is as important as billboards a mile from the highway. Keep your opponents simple and straight forward. Morale: Choose your setting
Fourth, concerning running combat. Personally for a first time GM I would recommend that you enforce that everyone has the same number of IPs. The reason is going from person to person, each with different IPs can cause someone to get skipped easily... For example:
Bob has 3 IPs and Initiative of 15 (11 + 4 hits)
Joe has 3 IPs and Initiative of 12 (9 + 3 hits)
Sue has 2 IPs and Initiative of 13 (9 + 4 hits)
Ann has 1 IP and Initiative of 11 (8 + 3 hits)
The Combat Turn would go like
Bob's First IP
Sue's First IP
Joe's First IP
Ann's First (and only) IP
Bob's Second IP
Sue's Second (and last) IP
Joe's Second IP
Bob's Third IP
Joe's Third IP
And that's not taking into account the enemy or changes to Initiative during the combat round. As you can see Ann only got to 'go' once vs. Sue's 2 or Joe & Bob's 3. Unless Ann is fine with this, it's not really fair for a starting game. Morale: Be fair to all your players
Fifth, make notes of what kind of tests to expect to ask for (Attribute + Skill, Threshold, Modifiers). It helps significantly keep the game flowing if you do not have to pause and look up a rule over and over again.... Morale: Do not let the GM screen be a buffering screen.
Lastly, read the Anatomy of a Shadowrun from the Runner's Toolkit. It does a decent job of showing a step by step process of a game session. Morale: Learn from other's mistakes.
I have been GMing for 20 years now and my players are always looking for ways to make me take my well planned notes and toss them out the window. And ultimately, that is the fun part of GMing. Not running the same session over and over again or having a bunch of players that follow every railroading plot line. That gets boring quick. Reward your players for ingenuity.
If fact, in the section on rewarding Karma, the point of "Right Skill, Right Time"... I let my players know that I read that to mean...
"Have and use a skill in a way that I, the GM, did not expect."
If I have a plot written around some player's affinity for Combat Biking or Urban Brawl, that doesn't really count, because I wrote the plot that way. But if the player manages to find a way to use his knowledge in 20th Century Comedians to distract the guards when a Con check failed, that is worth the point.
Ultimately, good luck