I typically start any game with a briefing. I will tell the players what they can expect of me the GM, and what i expect of them. I will usually give them some basic knowledge of the game world, the game mechanics and what focus i expect the game to take. I also typically tell them how likely their character is to die. Many of my players are new school, and therefore need to have the concept of player death explained to them.

Usually i pick a broad tone and style for the game. What will the game initially focus on. Will it be an exploration hex crawl kind of game, will it be solving mysteries and lots of npc dialogue, will it be a deadly dungeon crawl etc. This might change and is open to discussion . I am not a mind reader i cannot tell what each player wants to do with out them giving me some feedback. 

There are a number of things an rpg campaign must do to work. It must involve the players working as a party in some form. I'm not going to run 5 simultanous solo adventures. Therefore either the players need to come up with a rational as to why they are going to form a party and work together, or they will accept mine. I don't mind well established characters generating PVP conflicts due to things that have happened in the game world, but a level 1 character has low life expectancy and therefore value. So their wishes don't mean squat as an individual. 


You do have a direct choice as a GM between detail and player freedom. You cannot achieve both. Or at leat not if you want to hold down a full time job. I will typically start off with detail and limit player choice. Why? Because the players don't know the game world well enough to make informed interesting decisions, they don't know their characters wiell enough to really decide what they character ought to do. So often i will start with a short dungeoncrawl or investigation or something that gives a reason for the characters to be bound together, introduces the core game mechanics and the setting. After thi