I've studied and held rank in several martial arts. For a beginner I would recommend Shotokan Karate. Not because it's necessarily better than any of the others, but because of the discipline and philosophy a good instructor will include in the training. Plus it is a good and effective defense art. These factors are important, in my opinion, and Shotokan is the strongest in these areas. My second choice would be Tae Kwon Do, which is what I practice mostly. It would also not hurt to learn Tai Chi Chuan. Most people think of Tai Chi as a mild exercise, but it's also a fully developed martial art, just as effective as any other Chinese style.

I think your youth demands you learn the discipline and philosophy that Shotokan teaches. Once you have a few years experience, then you can make a reasoned judgement as to the other arts and which may suit you best.