Find A Good Yoga Teacher
Practicing yoga is not "one size fits all". Yoga instructors' approach vary. In style, experience and training. If you're young and fit, you will be able to handle a wide range of yoga styles and classes. On the other hand, if you're a 50+ years of age with super tight hamstrings or hips, just starting out, it may be better to start with individual yoga sessions with an experienced instructor.
The same thing applies if you have any injuries or physical limitations you're working with. Let your instructor know before the class, and don't be shy to ask if the class will still be suitable for you. If the instructor isn't able to offer an alternative related to your condition, that's a good indication the instructor might not a good fit for you. Keep going to find another one who is more experienced to suit your needs and your personality.
It is much easier and more enjoyable, to learn yoga, with all its detailed, subtle physical and mental aspects, from an experienced, qualified instructor, rather than from a book or video. Especially at the beginning, an instructor's presence and ability to actually see you, and respond to what you are doing, is essential for building your confidence that you are doing yoga properly.
You need to like your instructor, as a person, and as your mentor. The person who you're going to trust the most. To have a nice, friendly relationship with your instructor helps you to open up and be more comfortable to ask questions. After all - your yoga instructor or personal trainer look after your body and your health. The level of importance is similar to choosing a doctor. So choose wisely.