I was diagnose with DDD Also known as degenerative disc disease. Can anybody tell me what this is? And how can I fix it? Thank you

Not fixable. It's a result of living! As we all develop degenerative(wear and tear) changes by moving and weight bearing: some of us more than other. You can't reverse it but by optimize your weight, posture/ergonomics, and optimizing your core strength(think balance between your back and abdominal muscles) you can help minimize further damage to your spine.
DDD. This is breakdown in the structure, strength and support of the intervertebral disc in the spine. It comes due to age, repetitive use, trauma and injury. Although 31 is young for this, it is not uncommon in people who have been athletic or have sustained trauma. It cannot be fixed, but you can take steps to minimize the pain and effects of DDD. Maintain a normal BMI, do core strengthening regu.