Suggest Remedy To Make Knee Stronger After Injury

I fell on my left knee cap about four or five years ago on the edge of a stair step, and three or four times a month it feels sore. It feels more flimsy than my right knee and it make my leg want to twist to the left. I try to stand with my toes facing forward but my left knee makes them want to go left, when i try to make them point straight, i get this painful stretching sensation on the side of my knee. I want to know what to do to make my knee as strong as my right knee without surgery. I'm 15, male.