Yes, yes, yes and yes.
Find a class. An older instructor is better as they will have injured themselves a few times and know all the tricks to fixing the body. While a younger instructor will be more fun to watch, they will have less knowledge of technique.
After a few classes where you learn how to do it all properly, then videos/dvd's can help.
There are two main types - (from my experience) - "flow" yoga where you go from form to form relatively quickly and "slow" yoga where you perfect a form and hold it. My yogi practices Iyengar - also referred to as "furniture" yoga which is great for older, more beat up people. Despite popular belief, just holding some of the poses correctly is a pretty damn good work out. Yoga isn't all about relaxing and zenning and not working hard. After a good session you get some decent muscle burn, sweat and elevated heart rate.
I have 21,474,850 rep points...
My blog - read it!
http://www.citizenclass.ca