What should I do if my friend is suicidal?

So I've had a few suicidal friends these past couple of years and thankfully none of them have killed themselves. But I'm just scared if one of them actually do it. Like my friend's cousin committed suicide and she was only 10 and it made me cry cause it's so sad that a girl so young would do that. But ever since then I've been so scared that one of my Friends might actually do it. I've had a few suicidal Friend's the past couple years but none of them actually killed themselves. but I'm highly concerned that they might actually do it, cause if they do I would feel super bad cause I knew and it would kinda be on my hands cause I did nothing. So please someone help me out on this.