Knee pain when I fold my legs. At bay?

Hi doctor, I have been facing extreme knee pain (around the knee joints) when I fold my legs and unfold them after sometime. I mean, I'm getting more pain when I fold backwards and then unfold to normal position, and less pain when I sit crossways (with my legs crossed, casual sitting posture). I'm feeling more pain around the right knee, and also the left knee at times. I'm walking fine though. Rarely, I hear some crackling sound. But is this a serious issue that needs urgent medical attention? I'm using Homeopathy for other medical condition, but the doctor asked me to go for a knee cap. I don't know what type of knee cap to wear. Also, is it a viable solution? Or do I have to consult another doctor for this issue? Also, please could you suggest good foods that reduces bone issues and enhance overall strength of my bones? And what sort of exercises I have to do to keep my knee pain at bay? Please kindly let me know doctor. Thanks!!