Weightlifting and losing a lot of body weight has been incredibly helpful for me. A few years ago I was way overweight and had pretty serious back pain (due to a slipped disc) and knee pain (Osgood Schlatter). I started lifting weights and eating better and lost 50kg (110lbs), and over time my knee pain all but disappeared and my back pain is a whole lot better. I mean, it's no miracle cure, probably quite the opposite since it means a lot of work over a long time (and it's not like the pain is completely gone), but it's definitely helped.
Specifically, I think the exercises that have helped my lower back the most are squats, deadlifts and barbell hip thrusts, for which you can find any number of how-to videos on Youtube. I find conventional deadlifts pretty hard on my back, so I do sumo deadlifts. The goal is not only to strengthen the weak part of your back, but to strengthen everything around it and essentially relearn how to hold yourself and, y'know, do things. Just be smart about it and go very slowly; start with very light weights (or none at all), film yourself to make sure you're doing it right, don't increase the weight until you're confident you can do it without your form breaking down.
