There are lots of different kinds of yoga, and you will see huge variations between different studios, styles, and teachers. Some variations are just differences in styles. There are also variations in quality. In our opinion, "bad yoga" is yoga that leads to physical injuries or mental attitudes that conflict with personal growth, such as cult-following or spiritual egoism.
Most of us at Playpoi have cultivated some healthy scepticism towards the big yoga chains, like Bikram's. We believe each individual has their own specific needs, and should therefore find the yoga that is best suited for themselves. "One size fits all" schools that require all students to practice yoga one specific way seem strange to us.
The word "yoga" means to "unite" or "to yolk," as in "uniting oneself to the truth of the universe." Many people think yoga is just the asana practice, which is stretching, strengthening, and aligning the body and breath. Asana practice is only one of the eight limbs of traditional yoga. The others include breath work, meditation, compassionate action in the world, and other practices that have nothing to do with stretching and strengthening the body. We recommend the book "Light on Yoga" by B. K. S. Iyengar. It gives a good introduction to the complete philosophy of yoga. A wikipedia search can lead you to many more resources about yoga.
To get started, see what's being offered in your community. Many studios offer the first class for free, so you can try lots of different studios and classes to find what you like. Asana classes can range from being very peaceful with lots of stillness and meditation, to intense vinyasa flow which feels more like a kung fu workout! Both styles can be benefitial for both body and mind. The goal is to find a teacher who has sufficient education and benign motivation.
We happen to think that yoga should be sincere but not too "serious." Be suspicious of schools where people have big spiritual egos. Yoga is about dissolving ego, so an attitude of spiritual superiority over others is counterproductive. Look for happy people who care about the world and other people, this is the sign of yoga in practice!
We hope that helps at least a little. Enjoy practicing!