Getting in shape can include losing weight, building muscle definition and strength, and feeling fitter and healthier. Any kind of yoga, when practiced regularly, will improve your strength, flexibility, and muscle tone, but losing weight is a little tricker, since it also requires more cardio and healthier eating. See my full guide to yoga and weight loss for more info.
If you have never done yoga before and want to know how to start, the best thing to do is sign up for a series of beginners' classes at a yoga studio near you (ask your co-workers, relatives, and Facebook friends to recommend a good studio). This way you'll get a solid introduction to beginners' yoga poses. From there, it's great to keep going to classes as often as you can. If you want to practice at home, check out the 30-Day Quick Start Yoga Guide to kick start your home practice with a month's worth of yoga workouts.
While yoga is a great practice that is accessible to all (including seniors and the overweight), it is not a quick fix. It take time and commitment to lengthen and strengthen your muscles. As you study yoga, you'll be getting in shape, but that may come to seem like a side benefit to discovering a practice that allows for limitless development and personal growth.