First off, if you are in high school, a summer internship will look great on your colleage application. This will help you stand out among your peers as only a small percentage of student will intern during the summer.
Secondly, taking a summer job will help you choose what career you like or dislike. It is like "test driving" a career.
Internships for teens are more difficult to obtain because the competition is stiffer, but there are some steps you can take to land that summer internships.
The most important tip is to network and be vocal, tell everyone what you are interested in doing. Tell your network on MySpace and Facebook, ask your guidance counselor and teachers for help. Ask your parents to ask their friends and colleagues. Ask your friends parents and have them ask there friends. The more people you have working on your behalf the better off you will be.
You can also look online, as there are some good websites that advertise summer internship openings. Make sure you have a good cover letter and resume ready and start applying.