I don't know how am I supposed to DM a session 0 for my campaign.
My friends tell me that I am supposed to tell them what happens so they can create a character that fits the campaign, but I think that doing this would make the players prepared for what would happen and then be able to make characters who are perfect for dealing with the problems they are faced with.
A good place to start is this question and its answers, even though, somewhat surprisingly, "How to do this," is not explicitly called out as a question. This will still give you a good cross-section of opinion on what the thing is that you're being asked to do.
I can share my personal experiences of how I run a Session 0, though, as well as address your specific concern about player knowledge and optimization.
What I do is going to sound very simplistic: I get all the players together, ideally in the same environment (or one of the environments) that we're going to use to actually play. Right now that means someone's home with a nice big table to gather around. Back in the day, it meant the gaming shop where we could use a table, or a student center area when I was in college.
Then I describe in very high level terms the kind of game I am interested in running, and ask if the players are interested in that kind of game. This often starts out as something similar to a discussion of genre:
You can see this moves from the very general to more and more specific. It can also cover things like the tone of the game (Is player-vs-player a given? Is it taboo? How dark is this setting going to be?) and so forth.
It's really not a part of the game directly, it is a conversation, it is social preparation for the game, to make sure everyone is interested in the game and everyone comes prepared for the game that you're actually going to run. I cannot give you hard rules on how to resolve conflicts during a session 0, though, except a vague sense that since the GM is doing more work than anyone else he probably ought to get more concessions than anyone else. Other than that, this is social negotiation not unlike deciding on sushi vs pizza: It is highly dependent on the people involved.
As far as too much player knowledge goes, in my experience this is mostly a non-issue.
First, the point of the session 0 is not "tell the players what happens," but more along the lines of "tell the players what to expect." The precise genre of game you're running isn't (or shouldn't, in my opinion) be considered a spoiler.
Second, character-optimizing players will always exist, but this is fine. There are other ways to shut down optimizing players, and in the last recourse, you as GM are the one setting the challenge level of the campaign.
Consider another extended example: Imagine you have told your players you are going to run a 5e D&D game. Your unannounced concept for the game involves a background of goblin, hobgoblin and bugbear tribes unifying under the rule of Maglubiyet's priests, gathering other humanoid armies and eventually invading the "civilized" lands. In short, a very military, martial kind of a game with a lesser focus on enemy clerics.
If you keep this concept to yourself, you run a real risk of getting players designing characters with concepts and skills that are superfluous-- social movers, squishy sneak thieves, etc.
If you announce this, you're more likely to get a ranger who is, yes, in some sense optimized against goblin-kind. But that character also thematically fits the game, and may have backstory that explains why he is optimized against goblins. Or, alternately, if you have a player who is really set on being a social manipulator, you may at your discretion be able to insert some story lines and scenarios into the game which make that player shine.
Put another way, I'd much rather have a character optimized for my game and who fits well into the background, than to have a character optimized for situations that will only rarely occur. The second character is useless, and since optimizers gonna optimize, I prefer to co-opt them from the start.
(But note, even here you don't have to give away your entire campaign idea. You can keep it as simple as, "Lots of fighting with an initial focus on humanoid monsters.")
Your goal is to create a safe environment in which you can have and have fun doing things that everyone is interested in, for that you as a GM can apply a variety of tools based on attitude, organisation and cooperation with your players.
It is more comfortable to hold your session/campaign zero at the place where you will play (players will see how difficult attending is, see if it accommodates their specific needs, etc.).
Be explicit in articulating the goals that you and your groups of players want to achieve with your session/campaign zero.
Provide the context of your campaign and encourage your players to be open about what they want to do.
Compare your goals and find compromises and agree on the most important aspects of the social contract.
A social contract is not exhaustive, and often you will find out what kind of behaviour you don't want at your table while you play - because you lack experience in dealing with that group in that situation in particular. You want to have a mechanism for how to address issues at the table when they will eventually arise, agree on a method of conflict resolution.
You always want to address the following things (loosely in that order):
A social contract.
Game rules.
Table etiquette about topics that players feel uncomfortable with (distinctive from the general behaviour clarified in the social contract).
House rules, and the state of homebrew material, as well as optional rules.
Campaign expectations, Setting, atmosphere and mood.
Character creation.
Keep track of them by compiling notes, a checklist makes things easier. Write the important aspects down on a sheet of paper that everyone can access at any time (A pdf document to print out is my prefered method).
As a GM you want to moderate the discussion, but always keep in mind that you are not the authority of a GM out off the game. The rule that a GM is always right (PHB 6) does not apply in a session/campaign zero.
Ask for feedback on your setting and campaign, clearly, state what kind of feedback you deem as helpful.
Before you end your session/campaign zero ask if there are any issues left that have to be resolved before you can jump into the game, always act supportive and encourage.
Never act condescending. If your players say that they are uncomfortable with a setting in which people are raped by Goblins, then respect your players' stance.
Do not brush issues aside as trivial. If you want to run a campaign about slavery and players want to clarify what kind of slavery you want to employ, then answer them in detail.
Always remember that funny jokes about sensitive topics ruin the game. You aren't here to make fun of the sensitivities of your players, you aren't there to peer pressure them into situations that they are uncomfortable with to make it work at the table at their cost.
Always respect the boundaries of your players and adjust if necessary, if your concept of a Gritty Realism campaign with Goblins pillaging villages and raping humans to reproduce doesn't resonate, then change the campaign instead of your players. Maybe that campaign will work at a different table, but it is unlikely that you or your players will be happy with a version that is compromised so much that they barely feel comfortable enough to make it work at the table and you don't get to explore all the dark aspects of the world that really interest you.
Set due dates for the completion of the character creation.
Organise how and where you will play, how often you will play and establish an attendance policy.
Integrate the characters into your world and check if your players are comfortable with how you want to incorporate them.
I highly encourage you to set up your own checklist to make sure that you have taken care of everything, this checklist by the DawnforgedCast provides an excellent frame.
So if you choose to employ this style of how to GM, then your players will be able to create characters that they want to play which are also viable in your world. Maybe your players want to optimise their characters, if so, then they will tell you when you ask them what they want to do. I believe that there are no characters that are not viable in 5e, but if you want to run a campaign with optimizers, decided to roll for stats, and one of them has subpar scores all down the line, then you can discuss the challenge of optimising such a character. Perhaps encourage them to create a back-up character if they want to retire their character (by death or decision) or let them create a different character with new rolls if they aren't in for the optimising itself, but for the power fantasy.
