I'm 51years old and i really want to loose weight and tone up. my knees are bad and i really love step aerobics. what do you suggest?

Lower the impact!: The best exercise for a person is the one they'll do! but if your knees can't be trusted, you might want to try something with lower impact. I was amazed when i tried water aerobics; it is really quite a work-out! and the water makes you practically weightless. Losing weight will put less impact on your knees, of course. I encourage you to work with a personal trainer; even a few sessions can help.