You can't control the decisions your teenagers will make, but you can control your reaction to the crisis they're in. Make sure that your initial reaction isn't to scold or blow up at your teenagers, as they need your support at this time. Don't go into denial or blame your teen or spouse, either; rather, stay calm and realize that your teen is not the first person in the world to be going through whatever the crisis is. Tell your teen that you're there for him or her and that together you'll work through it. Yes, you can feel disappointed, frightened, outraged, or frantic about what the consequences will be; your feelings are important and valid, but you'll have to deal with them later, possibly with a professional or good friend.
You can acknowledge that your child made the right choice in telling you about the problem, even if it would have been better had you known earlier. Depending on what the crisis is, your choices can include availing yourself of the many organizations for support that deal with your teen's situation. Suicide help lines, Planned Parenthood, free legal advice, therapy, psychiatrists, and the police are busy every day with problems similar to or worse than the one you and your teen are facing. Plenty of help is out there, but you have to look for it. If you feel uncomfortable asking friends or neighbors for ideas, many resources are available online.
Another choice is to remove your teen from the scene entirely if part of the problem is the crowd that he or she hangs around with. A structured behavior program or specialized boarding school can let teenagers turn their lives around and make positive changes. If you adopt a "he got himself into this predicament; let him get himself out" attitude, your teen may learn his lesson, but he could end up with emotional scars, bodily harm, or worse.