Back Exercises/Stretch

Since you don't mention at all what is 'bad' about your back, your question is very difficult to answer.
If you have an actual back injury or medical condition, that question should be answered by a qualified medical professional, like a physiotherapist.
If you are just generally sore/weak, here are some suggestions:

• A sore back may be in large part due to a weak abdominal muscles. Your whole core works together, so focusing on just one area can be detrimental to general functioning.
• Be aware of your posture during the day, try to re-focus every once in a while, making sure you sit/stand up straight with shoulders down and back and a slight sway in your back, but without sticking your butt out.
• Large compound movements where you make sure to focus on your core being engaged (like dealifts) can be really good. Make sure you maintain good form.
• General strengthening for all of your core muscles like planks (and variations), side planks, roll-outs (on your knees with elbows on a swiss ball works well to start), cable chops, and leg raises.
• Back specific exercises like good mornings, extensions, and the superman