You know it. They know it. Teenage years are growth years and ones of great change. Not only are their bodies growing but also their brains - which continue to develop until their early to mid 20s. Teens need food, sleep, and exercise and lots of love and support – the same as for a child, but in different ways. Most things about their world are changing. Don't let your love be one of them. You might like to start with Parenting teens: Introduction.
Look for opportunities to talk about drugs with your teenager. Talk with your teens about ways they can so no to alcohol or other drugs, without them losing face with their friends. Be informed about drugs.
There are many reasons why people self harm. The most important thing to do is to listen. It is very important to encourage talking as that is a much better way to help the intensity of emotions than to self harm.