Exercise. If you don't have a history of an injury then most of the people i see that are you age and have low back pain have weakness and inhibition of their hip/gluteal muscles which will readily respond to appropriate rehabilitation.
See family doctor. Lumbago is a nonspecific term for lower back pain. There are many caused of back pain. You should begin your investigation by seeing your family practitioner. Most cases of back pain can be evaluated and treated successfully by your pcp. If needed they will refer you to the appropriate specialist.