After seeing the suggestions about sport/exercise which are less stressful, or even good for my knees, I chose squatting. From the suggestions, it can be done with minimal space and equipment at my home, which is also a plus. Especially I would try squatting with my own body weight so as not to give extra pressure on the knees. (Anyway I don't have a barbell.)
But Ryan Miller stated that "If you decide to try squats, just make sure your knees do not go more forward than your toes at the bottom of the squat."
I tried to mimic the squatting from some videos, but as I went down, I can only keep the knees right above the toes, is it okay? The knees tend to go forwards, but I can keep them back by focusing and downward movement in my butt.
Are there any key point to avoid injury, and facilitate squatting?