Become a better running back?

Hi, im 5'9, 150 lbs and want to start at running back for my jv team next season. I started at running back on my middle school team, and although I was ok, i think that i have MUCH rom for improvement. For example, i'm fast but i didnt actually use my speed when it came to exploding through holes and making cuts and jukes. I will be a freshman and although i am 14 years old now, I will be 15 when the season actually starts. Although I have always done pushups, I started actual weight training around late March, and I think i am in pretty good shape. What do I have to do to become a better running back?