Manage expectations. This is the first and fundamental mission you have as a Dungeon Master.
Before you even start character generation, have a discussion with your players about where they want to go with the game. Let’s call this “Session Zero.” Talk about whether they want hardcore, immersive, collaborative storytelling – that’s one extreme – or a miniatures-based
wargame with some throwaway flavor text – that’s the other extreme. Talk about character concepts, because it does no good if a player shows up with a carefully-constructed half-bear, half-elf bard/druid with fifteen pages of backstory when your campaign is set exclusively in an urban environment. If you’re going to use any of the optional rules, be very clear which ones are in use and which are not. Be prepared to negotiate with players who want optional rules you don’t, and vice versa.
When discussing expectations about game mechanics, recognize if you have players who haven’t mastered the mechanics of the game you’re playing. If you do, that player’s expectations about how the game will go will change as they get used to the game engine at work. You may have to have discussions about that later in the campaign. Make sure you reserve the right to have that discussion later on.
Talk about everything you can think of, not just game mechanics. Just as important is deciding who’s responsible for bringing the soda, what day of the week is best for play, what time is best, what happens if someone misses a session, ad infinitum. It may seem mundane and OCD, but if you want to reduce stress and strife later on, you must endure it, or you’ll be herding cats trying to get Jerry and Annie to stop going PvP because both thought it was the other’s day to bring the Doritos.
The point of this is to nip in the bud as many possibilities for strife as crop up later on. They will crop up. The wise GM manages expectations to reduce her crop of headaches.