Teens are always looking for new ways to assert their independence, to experiment, and to create a new identity. Getting a body piercing can seem to your teen a great way to achieve all these goals. In addition, since your teen is also starting to get more concerned with how he/she looks, the piercing can be seen as a means of making him/her more attractive. If you aren't supportive of body piercing, you should let your teen know what your family standards are on this issue well before your teen even brings it up. Setting expectations beforehand can go a long way in heading off potential issues and confrontations.
However, if your teen does decide to get a body piercing anyway, sit down with your teen and ask him/her a few questions. These questions shouldn't be asked in an aggressive or judgmental manner. Being genuinely interested in how your teen has made such a decision, and helping him/her walk through its ramifications, will go a long way with your teen and might increase your persuasiveness with him/her against the idea.
What is his/her goal in getting a piercing? Is it because someone else has one (e.g., a celebrity or new friend)? Where on his/her body does he/she want to have the piercing done? How does he/she think it will change how people see him/her? How does he/she think having the piercing might affect him/her in the future?
You should also ask him/her some practical questions, which can help you find out how well your teen has really thought this through and will make sure your teen is made aware of some potential risks of body piercing. Does your teen understand the risk of infection? Does he/she know how long it will take for the piercing to heal? Is he/she prepared to follow the daily regimen of washing the piercing several times a day with warm salt water, for weeks after it's done? What sort of metal will the jewelry be? Is it a safe material, such as surgical-grade stainless steel, titanium, or 14-karat gold? Help your teen think the decision through to the end, and you might be surprised what happens.