Make sure you find a school that understands and spends a lot of time on foot work first. Other than that, play other sports that require quick movement like basketball, eat well and do a lot of plyometrics. That along with an instructor that teaches actual movement, not that standing still or mechanical robot movement which most seem to do now (kata), you will be ok.

Masterful footwork and fluidity of motion. It really that simple. To flow like water, graceful, but strike with destructive force like a tsunami and with the speed and ferocity of a hurricane.