I do most of the pitching coaching for our team, and our players are your age. When they say to use your legs more, they are referring to you using your windup to propel the ball instead of your arm. When you keep your back straight and wind up tight, throwing your front leg around can propel your hips in the direction that you want the ball to go in and your shoulders will follow. Your arm is like a slingshot when pitching. Your release point and grip will change the pitch, but the power comes from your body mechanics and rotation. So, basically, using the weight of your leading led to propel your body into the pitch will give you much more power than throwing the ball. Keep your leg up and tight and allow yourself to “unwind” your wind up. Your back leg will usually end up in front of you after the release if you do this correctly. Once you feel it once, you will know the feeling and it will become natural to you.