Do you have a session zero before your campaign starts for real?
If you are not familiar, a session zero is when you and the players discuss the campaign that you are about to run and is intended to get everyone on the same page.
Of course, if you are running a one-shot, then this is unlikely to happen, but for a longer campaign, it can really help.
At the very least you need a way to tell the players what your campaign is about and how it might be different from others they have experienced.
While you can do this via email or a message board, getting everyone together, especially when they don't already know each other, can be a good thing to start players bonding and working as a cooperative team.
Expectations Management
Letting your players know what your game is and is not will be helpful to set good boundaries. You should also invite feedback about the kinds of games they enjoy and are less interested in.
Are they into roleplay or combat-heavy games, for example? Are there uncomfortable topics?
Player Creation
Character creation is definitely a place where you might want to set some boundaries.
How do you want the abilities rolled?
- What character types fit best in the campaign?
- Which books can the players use as references?
- Can the characters be connected in some way?
- What are the important factions and how do your characters relate?
Will you have your players come with their characters or generate them on session zero?
How Much Prep?
Some game masters go into session zero not even knowing what campaign the group will play, others come in knowing exactly the game they will be playing.
There is no set rule, but it does depend on how much information the DM has been able to gain about the players and their preferences. If you know already that your players want to be space bounty hunters then there is no need to come into game zero with a selection of Fantasy scenarios as options.
How much preparation must the players have done? Many DMs like people to generate characters at home to save session time and give them plenty of chance to come up with cool ideas, others need at least to see rolls done in front of them to stop cheating.
Cheating and Etiquette
Your ideas around etiquette, your rules on cheating, are also very important.
What DO YOU regard as cheating?
The World and Lore
What do the characters need to know about their world that the players may not be aware of?
Remember your characters have already had a life before your players take them over, they will have functioned in the world and will be aware to certain levels about the politics, history, geography, religions, and culture.
What Are the Characters Trying to Achieve?
Is the campaign centered around an overall goal? Is there a nemesis they need to destroy?
Your players might not have any specific missions, yet if you are playing a "tavern" or "job board" kind of game, but their backstories might have things that let you know their goals, fears, things they are trying to get or get away from.
House Rules
If you don't play exactly as written then you might need to go over any house rules.
I used to joke that in my almost 40 years of playing roleplaying games, I had never actually played any of the actual games because, from the start, every game seemed to be heavily house-rule-heavy. That was hyperbole for humour reasons but based on some truth.
You might be surprised how many house rules your players come with, too.
For example, some players expect a "critical" natural 20 to be an outstanding success regardless of combat or not.
- You might have a house rule that a role of a 1 on a spell causes all kinds of hilarious outcomes.
- Do you roll out in the open or behind your screen?
- How do you run initiative and movement?
- Do you plan to use XP or milestones?
- Is there anything they need to know around player death?
- Theater of the Mind or minis?
Bottom Line
You will notice a lot of the questions lead to issues of player agency versus pre-planned story. This will be a negotiation of sorts between the DM and the players.
Even if you don't have an in-person session zero, I encourage you to at least have the discussions!