Muscular imbalances can lead to injuries such as back pain, and can also contribute to your bad posture, which will add to the pain in your back, neck, and shoulders. If the muscles in your back or your core are weak, you are much more likely to suffer from chronic back pain or from an injury. This applies to other areas of the body too; if the muscles that stabilize a joint, such as the knees, are weak, then that joint will be weak and more prone to injury.
Depending on the area that is weak and at risk, simple strength and conditioning exercises can be done to build up your muscles. Start with some body weight exercises such as sit-ups and squats, and then build this up to incorporate some light weights. With a little research, you can find a plethora of exercises that target specific areas and provide results.