A good place to start is by talking: ask your friends or colleagues at work to recommend a teacher or studio or school. Even if you consider yourself in great shape but are new to yoga, sign up for a beginner's class. Also investigate the methods beforehand: some techniques are notoriously intense, such as Ashtanga, while others are gentle, such as Kripalu.
Before unrolling your mat, have an idea of what you're getting to. Get the details beforehand on the length of the class, the cost, what kind of dress is recommended (for example, for heated classes such as Bikram and Power Yoga, you'll want lightweight clothes that breathe), whether or not you need to bring your own yoga mat, and how large the class is. More experienced and popular teachers can draw huge, tightly packed classes, meaning less time to work with individuals. Newer teachers, while a little rougher around the edges, will have more time to give you personalized attention.
When it comes to finding a good teacher, make sure he or she not only has been certified to teach yoga but also continues to practice and study under a master yoga teacher. Talk to your teacher beforehand if you have any problems or issues, and look for a teacher who is patient and respectful.
While yoga can be challenging and will initially at least cause some pain, never perform a position in class that generates "bad" pain, especially in the knees, lower back, and neck. Talk to your teacher, ask for a modified pose, or assume a rest position. Never allow a teacher to encourage you to "work through" this kind of pain.
Also a good teacher will walk around the class, looking at the students' poses, making adjustments as necessary. Get a feel for how the students respond, whether or not there is camaraderie in the class, and if he or she offers feedback and alternatives. Also be sure your teacher incorporates some breath work, which is an intricate part of all forms of yoga.
If you find a teacher you like, it's best to study under that teacher as much as you can, allowing your teacher to familiarize herself with your practice. A good teacher will take a personal interest in you and your yoga by listening to your goals and hopes.