I'm a 24 years old and I suffer from a middle back pain for two days, what should I do to relief that pain?

Depends. How did it start? Trauma or some injury, or out of the blue? It could be something simple that will get better with anti-inflammatories and time, but if there was a specific injury or it doesn't get better in a couple weeks, you should go be evaluated by your physician. Sooner if you have any other associated symptoms like weakness in your legs.
Back pain. The back pain can be caused by muscle strain, spinal stenosis, ruptured disc, nerve impingement ... I recommend you try stretching exercise, massage and heat pads first. You also can try Ibuprofen over the counter as needed. If your pain persists, you should seek help from a pain management doctor. You might benefit from a comprehensive evaluation.