knee injuries

i am 17 and i used to play baseball alot until i moved to england but anyway during this i basicaly realy injured both my knees which would have been about three years ago, i like messed up ligaments in my knees.
well any way i am doin karate now and after going to a national competiotion managed to re injure one of my knees as it got kicked rather hard (still got third) but i was wondering what kind of exercises i can do to strengthen my knees without risking another injury. any info would be helpful. thanks