(A new GM flattered me recently by asking my advice on how he might best start off running his own games. He said he had an idea for something that involved the PCs doing battle against demons and orcs and ghosts to save a town, but hadn't planned much beyond that. What follows is the response I sent, which got so long that I figured I might as well just turn it into my bi-annual blog post!)
One thing I always suggest is that new GMs take a look at existing adventure modules to get an idea of the industry standard for layout and pacing. Writing a story is great, but when it's time to start rolling dice you want your players to be really excited and engaged with the sort of challenges you put in front of them.
That isn't to say that the story you want to share isn't a big part of the game, but you never want your players to feel like the things their characters do matters less than the narrative their GM is trying to tell. Keeping the right pace helps with that.
Also, keep a certain amount of focus. Demons and ghosts and orcs are all awesome, but do they have a reason to work together apart from being bad guys? Though all of them are chaotic evil, they all make for very different kinds of encounters, and probably wouldn't get along well under most circumstances!
Demons are probably the most challenging of the bunch to play, because they get all sorts of dangerous abilities - not least of which is the ability to summon MORE demons! Add to that DR and SR, along with generally terrific stats, and you've got some of the meanest monsters in the game. Further, they can operate in a myriad of different ways. While some of them aren't much more than rampaging monsters, others are subtle manipulators that you might never know are even demons until it's too late.
They also come from a place called The Abyss (in most campaign settings, that is), which is an infinite place of chaos and evil that will spawn infinite numbers of demons. It's difficult to see large amounts of demons bothering to work with orcs or ghosts.
Ghosts are also tricky, given their "nature". They are often tied to a particular location, and are incredibly difficult to destroy due to their rejuvenation ability - and once the heroes manage to destroy one for real, it's often the end of an entire chunk of the campaign.
Building your own ghosts also requires understanding of how templates work, and having a solid grasp on CR.
Orcs are easier to work with, but their tactics can make them just as deadly as the other two. They advance as characters rather than gaining monstrous Hit Dice, which means that you can essentially put your PCs against other adventurers of various levels - even equal to or higher than what the PCs are, sometimes!
I'm not saying that you can only ever use one sort of monster, but it's a good idea to hammer out where exactly the bad guys are coming from and where they hope to go.
Instead of rampaging nasties all over, what if a single Glabrezu (http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/demon/glabrezu) was banished from the Abyss because he angered his demon lord? The fiend (be sure to give him a cool name!) is amassing an army of orcs, ogres, harpies, and hill giant cultists to try and turn the home of the PCs into his own little piece of the Abyss...maybe he'll even try to become more powerful than his old master! Don't show all your cards at once here - in fact, don't even reveal the demon at all until you begin dropping tiny hints after several sessions in.
Maybe the PCs find his name but don't know what it means. Maybe a note here or there, or the orc cultists who worship him as a god dressing up with wolf heads on their helms and giant crab shells for armor. Knowledge: Planes checks, everyone!
Just off the top of my head: I'm sure you have your own cool ideas!
Be sure to challenge the players, but be prepared to let them be awesome. If they solve a problem you didn't expect them to solve, or come up with a solution to something that wasn't quite what you had in mind, try to think of a way to roll with it! That isn't to say that you should be a softie and keep from killing any PCs, of course. The game is only tense when people know they might fail.
I hope that this was useful! Let me know how the game shapes up.
Best regards
Kevin Mickelson
