You can be of pretty much ANY body shape in the martial arts. Both Sammo Hung and Jackie Chan trained in some of the same arts, with Jackie being of medium build and Sammo being a fat guy. Sammo also trained in Wing Chun, which little man Bruce Lee helped to make famous. Bolo Yeung, the big muscular fighter from “Enter the Dragon” was a champion TKD competitor before he ever hit the big screen.
Some arts actually make this known through their teachings. Tong Lun (Praying Mantis) offers a variety of tactics to suit a variety of fighter types, from small guys (“mouse running”) to big ones (“elephant strikes”). That’s part of the significance of the different animal styles and elements, that there’s some thing for pretty much everyone in the arts.
The thing is, you don’t even HAVE to be especially flexible. You can train as much as a runner in flexibility, just enough to be able to move as fast as you can without getting cramps, as a martial artist. Most southern Chinese kung fu systems (and the kenpo styles that come from them) focus on using low kicks so much that you’d never be taught to kick high in those arts. It’s really the ego of western stylists that has them training for such, not an actual requirement of their arts.
The best martial artists are NOT skinny guys. The best martial artists are those that can both give a good strike and can take one, as well as being able to get out of the way as much as possible. One good strike from a medium-sized man can end the fight for a man that is too thin, something I’ve seen a number of times (it’s kind of funny, actually). No matter what your style, you cannot block or evade absolutely everything; there’s comes a time, either when you least expect it or when you are under the weather, that you miss something.
Your most famous MA fighters have had a balance of speed and power, which simply is not found in skinny people. Chuck Norris was/ is of medium build, as was/ is Bill “Superfoot” Wallace. Joe Lewis was a little more toward the muscle side, certainly not a small man. Look at all of your famous MA fighters and their competitors to get a more realistic perspective of the range of people in the arts.
Lots of MA guys work out for strength. Most will do so through non-weight resistance training (such as resistance bands and push-ups/ pull-ups/ calisthenics) because the common myth is that bulky muscle of slower. So, by all means, do what you have to for building up in strength and muscular endurance.
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It all depends on your Personality. Visit schools in your area and you may be able to try out a class or two or just watch. Learning Martial Arts is a lot more than just getting in good shape, it is bettering yourself as a person. A good dojo will have stretching, strengthening and of course the techniques. There are many good web sites and books written and you can check out what the different styles are about. As you start you will be new and it takes at least 6 weeks before you can begin to see a difference and have it all make sense.
Just jump in there and try it out, most people think about it but never try. If it is a good dojo and instructor you will get in good shape and learn a lot of philosophy too. Mind, Body & Spirit
Open Heart, Open Arms, Open Mind… Go for it!

re: flexible
You stretch. There are many stretching exercises both western and eastern that help. Yoga helps. Tai Chi helps. Ki Aikido helps.
re: stronger
pump iron, and so some aerobic exercises to bring heart rate up.
re: martial arts
Most martial artists are on the trim, fit skinny side. Judo seems to collect the more rounded bunch. Judo and Karate seem to be shorter and stocky. Aikido, Kendo, Kenpo iaido seems to favor the taller set.
but manly do what interests you.

To be in a martial arts–all it requires is will-power. The majority of the people that goes into martial arts aren’t flexible or muscular from the start. You can be large, small, fat, skinny, lean, or plump. It really doesn’t matter much except of your adaptability and skill.
Because as they say, looks can be deceiving.

You don’t need flexibility, but it’s a plus. You don’t need great strength either, but it’s a plus. You don’t have to be in any specific kind of shape to training in martial arts.

I depends on what kind of martial art you intend to practice. For example if you want to practice Sumo, you have to be fat. If you want to practice judo or jiujitsu you got to be strong. But for other arts like karate, taekwondo, etc, any of these can work well.

stretching increaces ur flexibility, stretch every day and that will help a shitload mate. yeh ur 100% right the skinny blokes tend to do best in martial arts.. work on repitition rather than how much weight you lift..