This is a topic which you can probably find covered in countless forums online and everyone seems to have a different oppinion. And that's fine. What people seem to all too often forget any time this topic comes up though is that different people have different reasons for wanting to learn martial arts. I am a big believer that when you're taught properly, reguardless of what your initial reason for learning may be, you will end up getting more benefits and in different ways than you had originally planned. However, if you start out somewhere looking for something specific and that's not what's focussed on primarily or as soon as you thought it would be, I think a lot of people are more likely to drop that class rather than wait it out. Some times, that's not the wrong thing to do. Some times, you may find yourself in a class, run by a teacher, that is just plain no good. Or at the very least, it just doesn't fit your needs at that time.
That all being said, I'd like to take this time to highlight a few of what I think are the biggest different reasons that people get into learning martial arts and what they should be looking for in terms of each prefferance. I think that generally speaking, we can separate into four different categories. Those are: competition, physical fitness, hobby, and self defense. Let's break these down one at a time.
Competition seems to be the number one reason for people wanting to get into martial arts these days. I can't say that I share this motivation myself, but to each their own. If you are looking to compete, there are some definite things you need to be looking for and some you probably want to steer very far away from. Although I've never attended one, I'm told that there are some more classical martial arts schools which do not have sparring. Obviously if you're looking to get in a ring and compete with someone, you're going to want to know what it's like to have someone throwing punches and in fact landing some on you before you get in there. So, check on that ahead of time. Also, if you're looking to compete in any mma type events, you will most likely want to find a school or gym which teaches both stand up striking techniques and a decent amount of grappling on the ground. You could go to two separate schools, one for each type, but most of us probably don't have money for that and there are plenty of places around these days that are geared toward this specifically. Lastly, you'll probably want to make sure that if your would be instructor is not a winning competetor him or herself that at least some of the already established students in the school are. Aside from that, make sure you have good conditioning and good luck.
Next up is physical conditioning. If your entire goal is to get into shape, you may want to rethink taking martial arts alltogether and look into a workout program devoted to your specific fitness goals instead. I love martial arts, but when I go to class, I don't expect to spend half of it doing push ups and running laps (though I have before.... assistant instructors can be quite frustrating, but that's a rant for another time). I expect to be taught martial arts in a martial arts class. Yes, you can get your heart rate up and it can be good for you physically, but in terms of overall fitness, you may not find all that you want or need in a class that lasts an hour to an hour and a half and you can only go to three times a week (depending of course on where you go and what the schedule there is like). I make a point of keeping a separate fitness program up in my personal time away from class. When I can, I incorporate my martial arts into my work outs, but the two do not always overlap. The point is that you shouldn't go into a Kung Fu class and expect Tae Bo or go into a Tae Bo class and expect to learn how to defend yourself against a mugger. Personally, I suggest some sort of fitness program you can do from home. I've always had an issue with gym memberships, because most gyms aren't open 24 hours a day and if I'm at home, I don't usually have to wait my turn to use my favorite piece of work out equipment.
Moving on, I believe we are up to the category of hobby now. I hesitate to just say hobby here, because this could be for recreation as one might usually consider by hobby or just an unusual interest. I'm reminded of a friend of mine who always used to tell me that he got into martial arts, because he was so interested in the culture. Now, there are martial arts associated with pretty much every culture, but I'm pretty sure that at the time, he was referring specifically to Asian such as Chinese or Japanese. It doesn't really matter though. The point is that this was something different to him. It was a unique experience and a glimpse of something completely different from his normal life. And if that is what you want, you can't go wrong with a more traditional Kung Fu or Karate type class. There is a lot of tradition in these types of classes. They generally exhibit high degrees of discipline, concentration, and respect in these environments. Of course, as with any of these, there are always exceptions and no matter what your reason for learning, you should always thoroughly check out whatever school you are interested in well in advance of signing up for anything. Don't get roped into a three year contract for a class that you lose all desire to be in after the first two weeks.
Finally, we are at the self defense category. This is the one that a lot of people will probably disagree with me on, but quite honestly, they're probably just being blinded by their own preferences in most cases. A lot of people will tell you that the best way to learn to defend yourself is to go down to your local mma gym and start classes there. I'll tell you right now that is not accurate. I'm not saying that you cannot defend yourself using some of the same techniques you may see in an mma match. The truth of the matter though is that these people are not training for the street. They're training for a ring or cage. People say that the UFC is the closest you can get to a real fight. Well, I say a real fight is the closest you can get to a real fight. And anything that teaches you to fight in the framework of a set of rules, with a time limit, one on one, and with an option of giving up is not preparing you for a street confrontation. The first thing you need to realize which sounds simple enough, but a lot of people seem to have trouble with, is that there is no such thing as fighting dirty when you're defending your life. There is such a thing as excessive violence, but that's another story for another time. I suggest that if you are looking to learn real self defense, you look for a place and a teacher that deals with techniques such as eye gouging, groin strikes, head butts, knees, elbows, weapons, and multiple attackers. I still remember one day when I was waiting outside of class with some fellow students before we were about to start. One fellow, a rather large guy, was complaining about the stick fighting and knife work we did in class. He said that he didn't know why we needed it and that he wasn't planning on using martial arts in a real fight anyway. He didn't think a guy his size would need to use martial arts. I'm pretty sure this guy dropped out after less than a year. Maybe if he'd thought more about what he'd said that day, he'd have stuck around. Bigger guys might not be the first targets for would be muggers, but they can still be attacked. And size can be a factor, especially in the eyes of other people. So, if a larger person were to be attacked, what do you think the odds would be that it would be by someone their own size or smaller or that that person would be alone and/ or unarmed? Statistically, most attacks on the street happen with some kind of weapon and that's something that no mma fighter has to worry about inside of a ring other than maybe getting run into the side of the cage. I'm not saying a cage in your face feels good, but I'd wager it feel quite a bit better than a knife in your stomach. The best way to learn to defend against a weapon is to first learn to use it and the best way to learn to defend against multiple attackers is to practice against them and to learn techniques that are designed to work against more than one opponent. I know everyone in these mma tournaments loves the ground game, but if you go to the ground in a real street fight, you may not be getting back up ever again. Sure, you might get that first guy in a wicked arm bar, but his buddy just cracked the side of your head open with his steel toed boot. Some of the arts I've found to be the best for real defense are Jeet Kune Do, Kali, and Krav Maga. However, there is a lot to be said for most of the more traditional arts when it comes to real defense too. It's just a matter of finding the right teacher and putting in the work to get good at whatever you do. Ultimately, whether you can defend yourself or not is going to come down to you more than the art. But it all has to start somewhere. So, just make sure that you're doing the best you can to use the right tool for the right job. Keep your goals in mind when you decide to start looking at martial arts schools or gyms. It's like voting. You never really know how things will turn out in the end, but the more informed you are, the more likely you'll make a decision you won't regret later.