At age 14, your body is still growing and changing a lot. It's likely that this will fix itself in just a little more time. In the mean time, though, you could always try a little bit of weight training for some muscle definition. Walking is a good way to start. You can also do reps of lifting yourself up and down from your tip-toes. Squats might help, too, but you will be working a lot more muscles than just calves. These muscles are largely genetic, so look at your parents. They will be a decent indicator or how your own lower legs will continue to develop as your body grows.
Jogging long distance, definitely. I used to have the same problem and when I joined our school's running team by the end of the season I had very nice legs, not too thin or too muscular. I also got a nice tan, too, from running outside. Start off jogging half a mile every other day around your neighborhood and keep gradually increasing by a quarter of a mile until you are able to do five miles. Then you can work on speed. Squats and lunges help, too.
If you belong to a gym or can access one the stair master or eliptical, along with any of the leg machines will help muscle up your legs, but at your age you should consult a trainer for the best lower body regime