Adolescence is a time of expanding social interactions. Some teens navigate new social situations better than others. If you're teen is withdrawing socially, the first question you want to try to answer is if it's due to simple shyness or genuine social anxiety disorder, a more serious condition. Social anxiety disorder is characterized by intense fear of public embarrassment and judgment. The disorder is a fear so intense that teens who suffer from the disorder will go to great lengths to avoid social situations.
Shyness, as a personality trait, will generally be apparent early on in childhood. So if you're teen is truly shy, he/she was probably also shy as a toddler. If your teen's social reticence is consistent with his/her earlier personality, he/she is not likely suffering from social anxiety disorder. However, shyness can morph into greater social anxiety if a teenager is pushed too hard to become more social. If your teen is shy, you do want to build his/her self-esteem and provide positive reinforcement regarding any personal relationships he/she does have. But take care not to push a shy teen too far too fast. Instead, if you see incremental progress as he/she responds to your supportive efforts - - be happy with that.
However, if you are concerned that your teen may suffer from social anxiety disorder, you'll want professional assistance in order to get a formal diagnosis. If your teenager does have this disorder, there are various options for treatment, from talk therapy to medication to self-help techniques your teen can be taught to help him/her better handle social situations. Your teen can use breathing techniques and/or negative thought-assessment techniques to help get calm. You or a professional can teach your teen some coping techniques. You can also start exposing your teen to new social situations that he/she doesn't have to manage alone. In this way, you can help your teen learn social skills and gain confidence.