Tell them straight forward tell them how its bad for you a thousands of people die from drinking and drugs tell them that you want the best for them and the best thing is to not do drugs or drink
I'm not one to sugar coat things,so I would tell them about drugs and alcohol without glossing over things,and basically tell them this,"I cant keep you from drinking or doing drugs completely,but if you must try either,i'll try them with you,to help you adjust."