First, if they have shared with you that they are suicidal, do not take this lightly. You can contact The National Suicide Prevention Lifeline. Their number is: 1-800-273-TALK (8255) You may want to encourage them to call this number. You also have the option of calling 911.
If you suspect someone is a threat to themselves or others, you should call 911 or if you are a family member, you can go to the magistrate’s office and usually someone will come to your house and possibly get them admitted to the hospital.
You can also bring the person to a psychiatric hospital or the ER. This may be tricky if the person is in denial or violent.
Having a mental illness is nothing to be ashamed of. Although there is a stigma, please do not let this prevent you from getting help.
If you don’t think that the problem is urgent, you have other options. NAMI is a great place to start. They offer free support groups among information that can help you. Another resource is Mental Health Today. They have a list of phone numbers you can call for information on various types of mental illness.
If you are trying to figure out what might be wrong with the person, you may want to read Wikipedia, Psych Central , About.com or WebMD. There are numerous other sites that are very informative and you can use google to find them. There are also numerous forums and support groups online. Psych Forums is just one of those. Sometimes, it is better to search under the specific mental illness they have.
The most important thing is that you do something! Please do not wait until the person you suspect has a mental illness hurts themselves or others.