S.A.
Frist Congratulations!!! I agree with Jackie on the Pilates. As an exercise phyisologist, I always tell people to do all other exercises before doing any lower back exercises since it is such a vulernable area.
Try doing some lower back extensions, a staff memeber at the Y should be able to show you these as well as the following. Also squats and lunges will help as well since they build the support mucsles for the lower back. You can do these free standing with no weight or even with a ball.
If you still have questions, please contact me. Also the book, "Strength Training Anatomy" is very good at showing detailed exercises if there is no one at the YMCA to show you how to do them.