Some kids can't distinguish between right and wrong very well, and lie/steal in order to achieve certain results. As a parent, you must not permit your kid to continue doing so. Nor should you go ballistic on your kid. Instead, you must approach this from and educationist's perspective, and sieze the opportunity to teach him or her what is the right thing to do.
Sometimes, teenager cheat and steal in order to get more attention from their parents. If you're one of those parents, you need to spend more time with your son or daugther, to ensure that he or she feels loved and assured. Once that happens, it will be much easier to communicate your values, and to ensure that this bad habit does not become routine.
Of course, peer influence could be the problem in some cases. You should always know who your kid's friends are - although you shouldn't become a control freak nor prevent him or her from having healthy interaction with friends.
Ultimately, you know your son or daughter best. If you aren't able to control your kid, you should eventually seek professional help.