hey

Question
how do you get better at fullback,defensive line,middle linebacker

Answer
Dear Tieran,
Let us start with conditioning.
Try playing basketball it is a great sport to play and it will help you get into football shape.  Basketball can help your coordination, quickness, conditioning, and help improve your foot work.

You also might want to add handball to your workout schedule. Just like in basketball, handball forces you to run back and forth and side to side.
Handball can be of great help, to a football player, for all the same reasons as basketball.

Try playing these two sports, on regular bases, and you will get into fantastic shape. And your foot work will look great.

If you would like help with the technique needed to play any of these positions, please pick one and I will do my best to guide you.
Your friend, Coach Louis