Instead of trying to be the expert on your teenager's life, try to help them think for themselves:

Discuss the potential implications of poor behaviour choices. For example, "How does smoking dope make you feel the next day? So, if you feel like that, how's that going to affect you playing football?"
Help them think critically about what they see and hear. "So Paul said X: is that what you think?"
Help them feel that they can deal with life's challenges. Remind them of what they're good at and what you like about them. This will give them confidence in other areas of their lives.
Information is empowering. Point them towards websites that can give them information on drugs, sex and smoking so they can read the facts and make up their own minds.
Help them think of ways they can respond and cope. "So, when you feel like that, is there anything you can do to make yourself feel better?"
Encourage them to think through the pros and cons of their behaviour.

Talking to your teenager