I always do a Session 0 before starting a new campaign. I've even written up what I want to cover to share with DMs I've mentored and FB groups with new DMs asking. It covers the types of things I try to talk about, with some D&D-specific answers in parenthesis.
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If a new group
Social rules (respect the host's location; DM authority in game, everyone equal voice out of game; no PvP including stealing loot from the party)
If first time DMing for the group
Quick overview of DMing playstyle (RP heavy, tough encounters but I'm your cheerleader. Give it a try/improvise. Failing forward, and failure just leads to a different branch of the story. Heroic or at least good-adjacent party. Lots of paths – don’t assume you have to follow a hook because I put it there. Don’t assume something is level-appropriate. Mostly serious tone, humor is good but not silly characters. Death is on the table.)
Character creation (Materials available (published WotC books, may have restrictions/expansions based on setting), Point buy, starting level)
Expectations I have for players. (Try to make games, play your character, don’t use metagame knowledge, be respectful, no pvp including stealing and other non-combat )
What expectations players have for me
What expectations from players of other players.
Settling rules disputes (spend reasonable amount of time in-session (reasonable depending on severity), then will make a ruling to move forward we can continue talking about outside of the session)
When we play (every other X night. Will play down one, cancel down two unless it's at the last moment and people are on the way.)
Comfort zones/triggers (no rape; how feel on body horror and gore?)
Always
Overview of setting.
Rules and optional rules for this campaign (Point buy, using all WotC official that’s neutral, for FR, or for setting X, milestone XP, variant skills/abilities, possible variant rest, druids/polymorph do not need to have seen creature)
Death discussion (on the table/off the table/only named NPCs can kill)
Character brainstorming and creation. (Don't need to build every character, but everyone should have an idea of the party and where they fit into it.)
Players should have a discussion of how they want to do loot. (This goes hand in hand with the no PvP.)
Homebrew only
When I'm running a homebrew world, I've only got broad strokes done at session 0, and make the overview very interactive, get ideas of what the players are interested in, where they want their characters to be from and such, so I know where to detail out. I also encourage players to come up with details related to their characters - the knightly order they are part of, who is the beggar-king they trained under, what is the culture of the western dwarves like, etc. Grant some narrative authority with my veto power. Anything they build is something I don't have to, and has automatic buy-in when it comes up in play for the creating player.