I've been doing Iyengar yoga (a form of Hatha) for about three years, and I think the best way to start is in a class with a good teacher. You need to learn how to find the limits and recognize when they change (they will, weekly!). In yoga you use your muscles to pull and push your body into positions that will be challenging and can be downright odd. Others are darned difficult. It's really important to learn to do them safely. Go-go aggressiveness can be a hazard to your health in yoga.
Get a good teacher.