How do I make the mouse centered in Unity and how do I make it so that you rotate the camera by moving the mouse?

You need to create a Character Controller, and a script that controls the movements. There are some stuff on Youtube as well

If you want to make your own character controller, you can lock the cursor to the game (and make it invisible) by using Cursor.lockState = CursorLockState.Locked, then use Input.GetAxis("MouseX") for horizontal movement, which rotates the camera. Then do the same for vertical, if needed :)