That's a great question. Breath is a part of yoga, called pranayama. This is a really good write-up on it:
http://www.yoga.com/about-yoga/articles/pranayama-breathing/pranayama-the-science-of-breathing/
It should connect with your movement, so that you imagine it is leading, rather than the physical posture leading. Breathe deeply, filling your belly first like a balloon, then up into your upper lungs and chest. Try to make your inhale and exhale even.