Although I will elaborate on this theme in course of time, here is an answer, suited especially to more beginning students:
There are plenty of exercises for the lower back. You might want to try the following: (1) Yoga Pelvic Tilt http://my.yoga-vidya.org/video/relax-your-back-yoga-pelvic (2) Pavana Muktasana - this releases tension from the lower back http://my.yoga-vidya.org/video/yoga-relax-10-minutes-gentle
(3) Child's pose: relaxes lower back http://my.yoga-vidya.org/video/childs-pose-basic-posture (4) Erxercises to strengthen abdominal muscles http://my.yoga-vidya.org/video/strengthen-your-abdominal (5) Cat Pose to strengthen the back muscles http://my.yoga-vidya.org/video/relax-your-back-dynamic (6) Final relaxation http://my.yoga-vidya.org/video/shavasana-yoga-relaxation (you might want to put 2 pillow under your knees)
Start learning the first exercise for 1-4 days. Then add the second one for 1-4 days. Then add the thirs one, and continue like this. You should feel much better soon.
In the course of time I will give more infos on Yoga for the back.