First impressions are very important on an interview. If you walk in looking clean and well dresssed, your potential employer will know that you are mature enough to handle the job. When in doubt always dress up, not down and lean towards dressing conservatively. Your job interviews will probably be for a part-time job in retail, restaurant, babysitting or volunteer etc. so you don't have to wear a suit or a fancy dress. However, your clothes should be neat, wrinkle-free and clean. Your hair should be combed and neat and make sure your nails are clean as well.
Girls - What To Wear
For girls, wearing nice dress pants with a conservative top and jacket are perfect. Knee length skirts are fine, just try to keep it at or a little above the knee. It's ok to wear some color, just not too flashy or blinding.
Girls - What Not To Wear
Looking too casual won't make a good impression and you should never wear jeans on an interview. If you are wearing a skirt, make sure it's at or a little above the knee, no higher. Don't go for a sexy look, it's just not appropriate.
Boys - What To Wear
A nice pair of ironed khakis, a neat looking polo shirt and dress shoes is perfect for an interview. Going a step up and wearing a tie and ironed dress shirt says that you are serious and interested in the job.
Boys - What Not To Wear
Never wear sports clothes to an interview, even if the job is sports related. Even though you can look neat and fashionable in casual clothes, you will make a better impression if you dress up.