How can I help a teen who talks about suicide?

Common sense. It's politically correct to focus on the teen and the difficulties of adjusting to new roles & peers, but the teen suicides i've seen as a pathologist have largely come from appallingly abusive families. Find out what's really bothering junior, and if it would bother you too, consider what interventions might be possible or even mandated by law.
Listen. Listening and caring is very important. Help them see they need help. Get them help. Any teen that talks about suicide and has a plan needs immediate help. Get them to the nearest emergency room if you can. Most teens just need someone to care and be concerned about them...But suicidal thoughts are not to be taken lightly.This teen needs to talk with someone asap.