What would you do if you found out your 17 year old teenager has an std?

Get treated. It is hard for parents to find out their child has an std. It is important to get them treated right away to avoid long term complications. Find a doctor you trust to address contraception to avoid unplanned pregnancy and get her vaccinated against hpv. Try to have an open dialogue regarding your family's beliefs about sexuality and try to understand your teenagers perspective too.
Yes, but. Dr. Livingston has the correct response to this problem. Adolescents dislike, and in fact, are turned off by, being lectured at (to). So a non-confrontational dialogue weighing their point of view is paramount. Don't lecture. Listen. Give your child the dignity to accept their mistake and work to improve their ability to make good health choices.