Even if you get a video, I also suggest "
Yoga for Wimps: Poses for the Flexibly Impaired". It really helped me learn to modify yoga positions as I needed to.
I had a really bad yoga experience with a large yoga class. They did a lot of on-your-knees positions, and I just couldn't do them. I didn't feel like I could interrupt the teacher to ask for modified positions. So I tried and tried, through the pain in my knees, and ended up in tears by the end of hte class, and hurt for a week afterward.
DON'T DO THAT!!! If it hurts, stop. If you're taking a class, talk to the teacher about your concerns about your body ahead of time. She's heard it before. And she'll be able to help, and to offer you options that will work better with where you are now.
I've also had an amazing yoga experience with a smaller class. They went slowly, did positions I could at least try to do (mostly standing rather than kneeling), and I felt relaxed and energized at the end of it.
I keep that experience in mind when I think of the first one. The difference was the choice of positions, the size of the class, my confidence in speaking to the teacher, and her gentleness and approachability.