If you have not talked to your primary care doctor
about this issue yet, I highly recommend that you do so. Any persistent knee pain really should be evaluated by your doctor so that you can get to the bottom of the cause of the pain and get set up with a treatment to relieve the pain.
There are a few likely causes of your pain. The fact that the pain is worse with sitting and with climbing the stairs strongly suggests a condition called patello femoral pain syndrome (also known as "runner's knee"), which is an overuse condition in which the cartilage on the underside of the knee degenerates, causes pain with pressure and tracking over the knee cap. Treatments for this condition include anti inflammatory medications, rest from vigorous high impact physical exercise, and leg strengthening exercises.
Another likely cause would be osteoarthritis
of the knee, which is the "wear and tear" form of arthritis that tends to develop in middle age, but can develop earlier as well if there have been athletic or other injuries. Osteoarthritis is also usually treated initially with anti inflammatory medications.
Start by seeing your doctor at your earliest convenience!