What do I do?

What do I do?
I feel like I should tell someone. I probably should right? But then again I'm the only one who knows, and I was only told because they needed to let it out, they just needed to talk. They won't even tell their family for Christ's sake! I should tell someone. I really should. But they trusted me, and only me. They didn't actually specify for me to keep it to myself but it's kind of implied when you find out you're the only one that knows.
I really don't know what to do. At the moment all I'm doing is crying. And that's not going to help anyone is it? But I don't know what to do. I hate not knowing, it's the main reason I cry, because I don't know what to do. This time it's coupled with this secret. So I know I should be doing something, but all I can do is cry.
I should try and cheer them up. I should, I should definitely be doing that. But I can't sound any different, I'm scared that will tip them over the edge. And that's the last thing I want to do right now. I can't carry on like everything's normal either, not after what they've just told me. If I carry on like normal they might do it anyway, and I don't want that. I don't want that at all. I don't want them to just leave me because one day they're feeling a bit low.
What on earth do I do?
This has happened before with them, just not to this extent and the first time I was able to put it down to just a bad day, mood swings. We all have them at this age. But now I know it's something worse. Before it was puberty now it's depression. And what do I do? I have no experience with this! Why would I? No one should have experience with this, no one should have to go through this. Not me, Not my friend, not anyone. No one should have to feel that way. No one.
What do I do? I really need advice, but I can't tell anyone. I still don't know what to do. And I hate not knowing. Not many things can make me cry, but this is. Because I am so stuck. I have no clue what to do. And I'm terrified. I'm bloody terrified. But I can't tell them. If they found out how I'm feeling they wouldn't tell me if this happens again for the same reason they don't want anyone else to know. They don't want anyone to worry about them. I'm always going to worry now though. How could I not? You can't just say that and expect a person not to worry. Or be scared. Or even cry. You can't just say that.
What do I do? What would you do? What would you do in my situation? Would you tell? Would you help? Would you speak to them? Would you comfort them? Would you cry? Or would you just carry on like nothing's happened because you don't know what else to do?

What would you do if your best friend told you they wanted to commit suicide?