Played Middle Linebacker in H.S. and College.
As far as tips..Get yourself into the weight room and work your ass off, Seeing as how your in H.S. when it comes time to practice, dont be afraid to jump in wherever the coach needs someone, show him you want to play and will do whatever it takes to get on that field!!
Getting in the weight room NOW and make yourself stronger is the first step, your gonna take alot of abuse in the middle, work your neck,shoulders and legs!! And never let anyone tell you that you cant do it! I was told that my whole life, being 5 foot 9 I was a small middle linebacker, but I got my ass in the weight room and came out looking like a fireplug!! I didnt play at a big college, but hell I played!!
~Good luck, hope this helps.