I get pain in my sternum and sometimes I hear popping sound from the middle part of the chest.  it's been like this since 2 months. Help please?

Unsure. It sounds like you could have an infection in the sternum or may have had trauma/injury to the bone itself. I would recommend a chest x-ray to look at the breast bone/sternum. If the x-ray is neg you might need a more detailed study called a chest ct (scan).

Rib pain. Assuming no injury, your popping is coming from a cartiledge between your rib and the sternum. I doubt a fracture, since you would have remembered an injury.