In the session zero, find out what type of game your players like, combat heavy, roleplay heavy, mystery, heist, politics, etc. Let them know about any house rules and what you like as a DM. Give them an idea of how you want/expect your game to run. Give them an overview of the world and feel of the campaign that fits with what they, and you, like so they can create backstories and characters that work with the theme. I ask for a name and description of an NPC that each is friendly with, and one that they are rivals or unfriendly with. This allows me to weave these NPCs in at various points if I want and create ready hooks for side quests or sources of help/info if the players start to flounder.
I can think of tons of pieces of advice but starting out, this was by far the best advice I got and really helped me not only design a game everyone enjoys, but also it gave me so much to work as far as plot.