Recently someone interested in the martial arts asked me this now-timeworn question via email. He made mention of a catalogue of styles, almost implying that he should pursue more than one. I defined some very broad categories of martial arts: grappling, striking, competitive and so on. But the main idea I wanted to drive home was that he was asking the wrong question. How should I know what style is best suited for this young man? Really, a "style" is a theoretical construct. It's the school that should be the main focus of his query.
First and foremost the would-be aspirant has to do some investigating. Go out and visit some schools to get a feel for what it is you're looking for. Many people end up training at a particular venue because it's the closest one to home. The commute typically gets the highest priority. There's more to it than just that, and there are a confluence of other factors that need to be considered:
1. The reputation of the school.
Word-of-mouth is not only the best advertising, but fairly reliable. It's a good place to start. If you seek instruction for your child, talk to the parents from your community. Even if it's for yourself it behooves you to visit the school in question and watch a class in progress.
2. The instructor.
While you're observing a class, what kind of person do you see in charge? A style is only as good as the instructor who's presenting it. Not only someone who is technically proficient, but one who exudes the characteristics you would expect from a competent teacher: leadership, patience and empathy. A good teacher should be a source of motivation and inspiration. A sensei is one who has gone before on this journey, so (s)he should know what you're in for.
In the US, anyone can open a martial arts club, no questions asked. Realize that not everyone is qualified (or should be allowed) to teach.
3. The students.
A good school and its attendants should be like a surrogate family. How do these people behave? Would you want them as neighbors? How do the students interact with each other? Personality and even occupation play a factor in how you'll fit in at a particular school.
I've noticed that certain styles tend to attract specific classes of people. Intellectual, white-collar and artistic types seem drawn to internal styles such as tai chi and aikido. The young guys love MMA. The blue collar working class gravitate towards karate and kempo styles. Of course these are sweeping generalizations based on my observations.
4. Method of payment.
Avoid schools that offer contracts. You never know how things will work out, and once you put your signature on something it can be very binding and difficult to get out of. Look for a place that charges by the month. I used to train at a school that had a pay-as-you-go policy (I was paying $4 a class, although that was many years ago). These days corporate schools charge up to $150 per month and expect you to sign for 6 months to a year's worth of lessons. Payments are then automatically withdrawn from your checking account. Welcome to the wonderful world of McDojos.
This list is by no means exhaustive. My point is that your school, its attendants and especially your instructor(s) are far more important than whatever style you train in. Visit some schools you may be interested in, and listen to your intuition. Choose wisely.