Sitting at a desk every day causes our hips flexors to become over-active and tight making our glutes under-active or 'go to sleep'. This imbalance causes other muscles to come into play to help assist (namely our back) which can cause that niggling ache.
By daily stretching you will help increase your body's range of motion, increase blood flow whilst strengthening your muscles.
Working out at least 3 times a week, using either body weight or weights will also help strengthen weak muscles such as your core and glutes, which will help with your posture to help alleviate back-ache.
Not sure what to do, or worried about injuring yourself with incorrect form, then drop me a DM to see if or how I can help you.