What do you say to someone who is talking about killing himself?

One of the responses to the question, "What is something you've talked about doing for years, but still haven't gotten started on?" (the question of the day 2 days ago) was "Killing myself." When I first saw that, I thought that the guy who said this couldn't be serious. But then, just in case, I said, "I hope you aren't actually considering doing that." He responded, "Yes I am." I didn't know what to say to him. I suggested that he try to get some help, but he said that it just didn't seem worth it, that his life just wasn't worth living anymore.I know next to nothing about this person. All I know is that he is an EP user, male, ages 18-21. I know that this can be a difficult time in anyone's life. This is when we're trying to figure out what we want to do with our lives. We know that we need to become independent, but we don't always know how. It can be a little scary. I also know that this person isn't really my personal responsibility, but I want to help if I can.