Just talk to them about stuff if you can do it in a non-cringe making way. Get your facts straight. They will encounter these things and will probably know a lot more than you do about them, so get independent information. If you start talking what they either know or just perceive as rubbish (i.e. smoke one spliff and end up adicted to heroin!) they will ignore everything you say. So on the drugs issue, look at some official governmental info (the 'ask Frank' government campaign in the UK is very good) and don't get hysterical about it. It will also help you find out how to look for signs of drug use but again, really really be careful and don't get carried away and the majority of teenagers act like that with parents anyway (moody, secretive, mood swings etc).
On the sexual side, I guess that maybe your beliefs are that pre-marital sex is wrong and there is certainly nothing wrong with imparting that to you children. Be realistic at the same time though and recognise that they may not share this belief, so make sure they understand about contraception and safe sex and make it a subject that they can talk to you about, rather than something they have to keep hidden from you.