I probably messed up something and I can't type tilde and brackets in my terminal. I'm on Mac OS X but I thought this has something to do with Emacs that I use just from yesterday for an online course. What can I check?
You can use Emacs in the X11 environemnt. Then you can forget about the Meta-Key in default terminal.
I solved unchecking the "use option as meta key" in my preference's terminal. But I fear it's not a solution because I need that for emacs.
You should try iTerm2. It's generally better than Terminal.app. Sometimes you need to type
reset to reset the terminal, it could be that too.