Let me address a couple of the things you broach in your post. First, the "where to start" is not nearly as important as the starting itself. I'd recommend classes with a highly trained, skilled yoga teacher. One is most likely to find such a person at a yoga studio (rather than fitness facility) though it is not always the case. Some very qualified teachers teach at gyms and some very careless teachers teach at studios. Generally speaking however a yoga studio tends to care more about what is being shared than a gym does.

If you opt for classes at a local yoga studio it is completely appropriate to call them first and ask them a few questions. For example - teacher training and certifications, on-going training, style of yoga, climate offered for same, and then finding out who the senior teachers on staff are. Then go and take 8 classes with different instructors. If the teaching moves you closer to your purpose and you see a teacher living a way that appeals to you (ethical, honest, kind, compassionate, authentic, with integrity) then you've found your start.

While you may of course lose weight by increasing your activity level AND that increase may include yoga, please refrain from thinking of Yoga as a weight loss method. It can be a by-product, yes. Frankly there isn't enough caloric expenditure in safe yoga classes. In many unsafe classes you're likely to experience high heat and very rapid movement. That's a workout you can get in so many other ways: jumping up and down in a sauna, rowing, cycling, elliptical or stair master machines. Yoga should offer you something much, much more.

There are very sensible, legitimate ways to shed weight at a reasonable rate, 1-1.5 pounds per week. I'll not go into the entire formula but it requires calculating your BMR, knowing your percentage of body fat, determining your activity level, discerning the appropriate caloric intake, eliminating everything from your diet that does not fuel you (meaning anything labeled "fat-free" is out), and sticking to that caloric intake while apportioning protein, carbohydrates, and fat in the right balance.

Nearly all yoga styles are rooted in hatha yoga, yoga’s physical discipline that focuses on developing control of the body through asanas or poses. In Sanskrit ‘ha’ represents sun and ‘tha’ represents moon. Hatha represents the duality in life – ‘yin’ and ‘yang’, masculine and feminine, darkness and light. It leads the way to balancing these opposing forces. It is the yoga of physical well-being. The difference between these Hatha-inspired styles often lies in how they combine (or don’t combine) elements of asana movement, alignment, intensity, breathing, pranayama (breath control), meditation, relaxation, chanting, lifestyles, etc.