Call your county courthouse to determine what court handles matters of commitment. If the person you’re concerned about is a minor, this will generally be the juvenile court.
Contact or visit the appropriate court. Ask for a petition to have someone committed. In some states, you must be the spouse or a blood relative of the person you want to commit. In other states, anyone can make such a request, but there are usually restrictions, such as that you actually witnessed behavior in the recent past that makes the person a danger to herself or others.
Complete the petition. It will most likely ask you to explain the behavior you witnessed that has you so concerned. Give as much detail as possible. For example, your parent might suffer from Alzheimer's, yet insist on living alone. If you visited her recently and found all her stovetop burners turned on and burning for no reason, this would signify dangerous behavior. Describe the scene; don't just say you think she's lost her faculties. Depending on your state's laws, you might also have to gather and submit a doctor’s report or statements from other corroborating witnesses.
Take your petition back to the court and file it with the court clerk. Some jurisdictions will immediately schedule you to speak with a judge, so you can orally detail why you think the person should be committed.
Attend the court hearing for commitment, if you’re required to give testimony. Some states might require you to appear and repeat under oath what you've already told the judge. Other states may not allow you to attend the hearing because the proceedings are confidential. If you can’t attend, you’ll receive notice of the outcome.