When shopping for a class, ask if you can observe a session first, and see if the instructor has a style you're comfortable with. Sessions generally last 45 minutes to an hour. Most begin with some gentle warm-up exercises, followed by controlled breathing routines. This loosens the body up for the postures that make up the bulk of a yoga workout. Ranging from neck-rolls to elaborate poses, the idea is to stretch the muscle groups and gently compress the organs in a specific sequence.
Class usually ends with a period of meditation.
Ideally, you should plan to attend classes at least once a week. Your instructor might also suggest exercises you can do on non-class days. Some can also give you nutritional and lifestyle advice that can help you towards maximum health. Especially for beginners, it's probably best to work with an instructor to make sure you're moving and breathing correctly and safely. That said, there are numerous videos for every skill level.