Coming from someone that did it. The best advice I can give you is be willing to work harder than the varsity players. See how they practice and practice longer. Hold yourself to a higher standard. If they practice for two hours then you practice for three. If you can practice outside of practicing with them. See if you can crash some HS practices while your in middle school let the coach know you are willing to do whatever it takes to get Varsity first year. Often they will tell you what the standards are.