You've made a decision to train in the martial arts. That was the easy part. If you glance through the Yellow Pages of any metropolitan phonebook, no doubt you'll find dozens of schools offering instruction in dozens of styles. Each year, thousands of Americans begin instruction in a martial art. Decide what it is you desire most from your training.
Most people train in the martial arts for one of two reasons.
Most styles of martial arts can provide both benefits, at least to a degree. And well they should. If they don't, I suggest you continue looking. However, many styles and systems differ widely in their approach to both fitness and self defense. Many schools teach systems that are tailored for competition.