I am 41years old. i did not have problems with my knees until i moved in my apartment because i have stairs. what can i take or do to stop?

Several things to do: I suggest seeing your doctor to consider an X-ray of one or both knees. At 41, there may be a component of arthritis present. Aside from that, if you are carrying a bit of extra weight, work on losing bodyfat. You can also work on strengthening the thigh & hamstring muscles which directly impact the knee joint. Also consider anti-inflammatory supplements like fish oil, curcumin, and boswellia.