What am I supposed to do if someone tells me that they are going to commit suicide? I haven't come across this yet, but I want to know what I should do to protect them...

Meaning is something that is created for each person individually, depending on their character, upbringing, personality and all that. There isn't really any single thing you can say since people have suicidal thoughts for entirely different reasons. You can tell them to visit a specialist, I suppose, or tell them you'd be sad if they committed suicide? Or you could just listen to them and tell them to tell you why they are having such thoughts. None of these are guarantees, though. Some people are so deep in depression that they don't listen to any arguments.

visit a specialist

Listen and just listen.
Stay with them.
Seek a professional.

#1 slap em

That's all I got :/

 no