start going to a local gym, you usually want to start with fitness to get warmed up, i dont use weights that much because i am only 15, but do core exercises and start to built up to legs and arms, i am only 15 and i have made Queensland and Australia gold, my program has helped me very much.
play sport
Practice long distance running and sprinting. I usually run two laps around the field, and then when I'm feeling exhausted I SPRINT one more lap. You have to push yourself that much more, just like in a game when you're exhausted and need to get your butt to a ruck quickly.
Just keep playing. Keep practicing and build muscle/strength outside of the playing field. I don't know about your coaches but my coaches put players in depending on how often they come to practice. So go to every practice and strengthen yourself, your ability, and the possibility of playing. ~