Good exercise routines for a out of shape 13 year old with bad knees? Read more..?

It may not be so much your knees, but maybe your legs as a whole? If that's the case (because I honestly don't know how to strengthen knees), you should start with you calves because they're key in running. All you have to do (to build up your muscles on your calves) is hang on to a wall and go up and down on your tip toes. Intermittently change from being on both legs to one leg at a time. It helps to count a couple 8 counts to music for each time you go from two legs to one leg. I REALLY hope this isn't confusing, but you should do this for about 10 minutes. Also, be sure to stretch your calves after wards! Also, do some wall sits for your thighs for about 1 minute for a couple times a week. It's a good idea to start building a better abdominal section so that you can hold your core while running back and forth while playing badminton, which you can achieve by doing crunches or sit-ups for like...10 minutes a day? Good luck! Sorry for the lengthy answer! I hope this helps!