The most important thing to remember when picking up a DM Screen for the first time is:
Never tell the players No, they can't do something.
Let them do whatever, then crush them mercilessly using clever tricks or the world environment. Nothing better than having a party member gloat over the ring of spell storing they stole from a high ranked mage, then later when the ring is used, it yells out STORING - S - T - O - R - I - N - G. Or a Hammer of Dwarven Throwing that throws the dwarf attemptin gto weild it as a missle weapon. If an action is pointless, or disruptive, or something like that, you can always say "sure, give me a roll" and then say "sorry mate, almost, but your target was a few points higher than that".
2nd most important thing to remember is: If the rules get in the way of the game or the fun, ignore them.
At the end of the game, your players should be bragging to each other about how they beat that nasty trap / monster / arcane puzzle by discovering a small but crucial flaw, or how they almost got thrown off a cliff by the ogre yet thanks to some very fancy foot and hand work, they saved themselves or even did some fancy ninja-like thing and returned the favour.
Finally: Leave real world issues at the door.
Sometimes, gamers will get shat off with each other over something. The gaming table, however, is a sacred space, and is not to be tarnished by real world issues. If you've got a problem, don't inject it into the game. If you're the DM, and you're pissed at one of your players, smacking him down in game is going to cost you a player. Leave it at the door, sort it out after the game, whatever, but don't bring it into the game. Same goes for your players.
One little thing I used to do to help keep players focussed on the game was to institute a good call / bad call system.
If someone said or did something witty, depending on its relevance to the game, and how much it made everyone laugh, I'd give them either 50 or 100xp. If it was an out-of-game call, I'd award 25 or 50xp. Having a table full of quick wits, the calls had to be exceptional to receive an award, but in-game relevant ones were handed out a bit more frequently than out-of-game ones.
The bad call points worked a bit different. If someone said or did something that made the entire table groan, they were given a baddie point, which had to be marked down on the character sheet. These things were nasty to have, as they conferred in game penalties.
Every time you got a baddie point, you had to roll a d10-(number of baddie points).
If you rolled over the number of baddie points you had, you were fine(ish). Under, and your character died of something stupid, like tripping on a frog and crushing your skull in a flower garden.
Whlle you had any baddie points, you suffered a negative penalty to all of your rolls equal to the number of points.
If you managed to collect 8 of them, you suffered the Wrath of the DM Continuum, and your character sheet was torn in half in front of the table, and you had to go roll a new one (lvl 1).
the only way to get rid of baddie points was to get good call awards (in-game only). The number of awards you needed to get to lose 1 baddie point was equal to how many baddie points you had (so if you had 4, you had to get 4 good call awards to go down to 3).
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If you've played some warhammer before, I'd suggest starting with 4th Edition, then when you're comfortable, track down some 3.5edition books. I do have to say that I don't see any reason to not stick with 4th ed, it's just as easy to run a story based game there as it was in 3/3.5e. I restarted my decades-spanning Forgotten Realms campaign (which has always been far more of an rp than a combat focused game) a couple of months ago, and if anything, the combat is streamlined and quicker, allowing for far more story and character development time.
Anyways, sorry for the mega-post, but I tend to get a bit babbly and overexcited when extolling the virtues of one of my favourite hobbies, or offering advice to first timers. Get stuck in, let the dice fall where they may, and have fun!
