Like you, I am overweight and very inflexible. A couple of years ago I was at a conference for people with weight/health issues and took a chair yoga class. There were people doing it who were in wheelchairs! This year, during physical therapy for a hip/back problem, I discovered yin yoga. It is a method of yoga where you go into a position and hold it for 3-5 minutes. It really works on letting go of the tightness in your joints and tissues. It has really helped me. I live in a rural area and going to classes would mean a lot of driving, so I'm using the internet. Check out Ester Eckhart on YouTube, Sarah Power's book "Insight Yoga", and Bernie Clark's website yinyoga.com. (I'm also fortunate that my massage therapist teaches yoga, including yin, so I do have a professional to ask questions about what I'm doing.) I agree with the suggestion above that you talk with any teacher before you sign up for a class and make sure it is someone who understands your situation and will work to make you comfortable in class.