Education. This is a natural part of life. Understanding consequences and dealing with them logically is occasionally beyond the teenage mentality, in which no adults understand their angst. But that does not mean that we should not try. There are those, however, who disagree, and feel that keeping people in the dark is a way to have them appreciate the light.
Educate & Discuss: Consequencies of wanted & unwanted pregnancy. How this will affect their future & affect all their furure plans & unfairness to any resulting child & to teenager's parents. Also explain how their hormones will naturally give rise to feelings of sexual attraction & possible need to consumate. Discuss peer pressure & how to avoid. Talk about safe sex, contraception & stds + avoidance. Be honest.