The first thing that you need to understand is that the warning signs of suicide are different for people of different age group. Do not expect a teenager to show the same signs as a grown-up or even an elderly people. Make sure you are aware of these signs by researching them on the internet, or speaking with a psychiatrist.
No manner how minor the suicide threat or attempt seems, be sure to take it and the accompanying signs seriously. You do not want to overlook something that is staring right at your face. If a person talks to you about committing suicide in a joking manner, take it seriously.
If you notice signs or hear the other person mention suicide, do not hesitate to ask if there is something bothering him. Asking questions will encourage the other person to talk his problem and how they are affecting him.
Be willing to listen. Do not start giving him lectures on how wrong suicide is and that it is a sign of weakness. There is a good chance that this will push him even deeper into depression and anger, thus bringing him closer to committing suicide. Just listen and encourage him to keep talking. Comfort him if he starts crying at any point.
After you become aware of the issue that is making him depressed, ask yourself if it is something you can help him with. If the issue is something that has made the person really depressed and hopeless, he may need to see a psychiatrist. Make arrangements for his appointment with a psychiatrist without wasting any time. The suicidal person may not be willing to visit a psychiatrist, but it is your responsibility to convince him to do so. If needed, get other people involved in convincing him to go through with the appointment.
Continue to support the person during his treatment. Follow up to see if the treatment is having a positive effect on the person and pushing the thoughts of suicide out of his head.