Having a yoga instructor can really help to boost your yoga practice. They will demonstrate the asanas (postures) so you can really see what you need to be doing and they will help you by watching what you do and making small corrections to keep your body aligned and safe. However Yoga is a wonderful tool for stress management that you can learn on your own with the help of a DVD or book.
Yoga and India aren't strangers, yet we find Yoga's popularity suddenly spiking after it's embraced by the west. It has become the thing to do to be fit with celebrities swearing by it. It is as if Yoga is a 'novel' concept or is reinvented by the west. After it has become the 'in' thing, there are two sets of people- the ones who do it and others who don't. Most of people who do it, adopt yoga for life while the others don't want to do it because of the myths around it or don't know how.