Easy way to lose about 10-20 pounds?

It doesn't have to be fast, at all :). I'm willing to take my time. I already know about diet and exercising daily but my question is, what sorts of exercises could I do? I don't want to gain muscle, I just want to drop a few pounds. Again, I don't care if it takes like 6 months, I just want it gone. Are there any easy exercises I could do without going and buying equipment? The reason it needs to be easy is because I have really bad joints and if I exercise too vigorously I get all messed up and I'm out of commission for a while. I'm a female, 13 if that helps. Thank you :)