Thank them for trusting you and having the courage to tell you.
Respect their confidentiality and assure them you will keep it.
Provide the teen with support, care and empathy and gently ask who else they have told. It is not your right to tell others. Unless, of course the teen is in danger.
Assure them you still care for them. Some teens fear that when they come out, they will be rejected.
Learn about organizations, support groups and books that can be helpful to the teen and share this with them. Refer them to www.transyouthequality.org
Being a good listener shows respect and being heard is part of the healing process.
Attend workshops, read journal articles, know local resources and be openminded about learning more. Encourage the teen to build community within the trans youth groups you know of or in LGBT groups regionally.