Have you been to the doctors to maybe check for arthritis? It's probably nothing but it's obviously bad enough for you to notice, so it might be worth getting it checked out. Fish oil capsules and foods high in Omega 3 are meant to be good for keeping joints flexible so that could help you. As for exercise, anything that's low impact would be best. So things like swimming as the water holds you up, cycling, even walking as long as you don't over do it. Things like high impact aerobics or jogging are probably not a good idea.
You might also benefit from buying a sports bandage, so that when your knee does feel achy, the bandage can give you extra support x