how to start training yoga?

if you download itunes for free you can get free podcasts with lessons on yoga its quite good. www.itunes.com

The best thing would be to join yoga classes. The instructor would teach you correct breathing and all the basic exercises. It is also nice to do something like this with other people until you grasp all the basics. Yoga is fantastic for the body mind and soul.

I've tried yoga. I just bought a Yoga video that was marked "for beginners". It started very simply with a few stretches and poses such as the "warrior" pose, and then gets progresively difficult. I haven't really managed to do the difficult stuff, but I don't mind. The easy poses are enough to strech your muscles and correct your posture.

sign up for a class, or buy a video and go along with it.

I highly recommend that you not try to teach yourself at home. There are some wonderful guidance DVDs out there but I believe they are better used by people seeking to augment their practice. The practice of yoga affects all of the various systems of the body, (inner, spiritual, digestive, mental, physical, etc.) and you really want to find a teacher that can coach you in understanding all of this.