I completely confer with FreeManDan. Good mornings, deadlifts and the like are a good way to reinjure your back as the lumbar musculature that supports stabilization of the spine has not been strengthened. You need to be focusing on the muscles that you cannot see with your shirt off, and not the ones that you can. FreeManDan's charts are the right exercises to perform, but in the reverse order of the pictures. Start on the floor just pushing yourself up like a push up, but don't let your pelvis come off the floor. Then you can work up to hyperextensions off the floor and then the machine. Once you have gotten that, follow it up with some of the machines in the gym. You should concurrently be doing core work, not necessarily abdominal work. You can find several examples of this by doing an internet search for core exercises. Slow and steady, don't rush it. It will just cause you to reinjure yourself.