57 is quite light, don't focus too much on getting bigger as you still have time to grow, a good number 8 is the best tackler in the team and is never afraid to run the ball straight and hard. Take these into consideration. No. 8s these days are also very fast and fit so Id focus on these two factors as well.
have fun. what is it about playing number 8 that you enjoy. is it picking the ball up from the back of the scrum and running? is it tackling? is it support play and linking with the backs? find the thing you enjoy and get good at that. its a fun game. play with a smile on your face.
Well try and watch heaslip on youtube then try and replicate what he does as he is awesome as an 8 hes fast strong and a good ball handler his tackeling is pretty damn good too but to be a great 8 you need to be able to read the game and know where your team mates are going to be for quick off loads and blitz rucking other than that keep fit get stronger and faster and with your age you could go far lol good luck
Run Fast and Hard