Three Weeks Ago I Fell And Hurt My Knee. I

Three weeks ago I fell and hurt my knee. I was on crutches for a week and moved to a cane for a bit after that. I have had the knee x-rayed and there are not fractures. The doctor that I saw felt that my knee cap was severely bruised.
My question is, when can I start to exercise to strength my knee and what would be the best exercises to do?