I agree with all the above advice but my two cents worth would be, have a good idea of how a steam loco works and what all the different things that hang underneath and are bolted on top do. i'm thinking injectors, ejectors, vacuum bags, brake hangers. All those little details that you need to pick up on that will lift your model to something really special.
You say several times your a young person and the above may be doing you a disservice. I hope you have already hoovered all this stuff up, I expect the lads will point you in the direction of another buying spree! Good luck enjoy the hobby and please post your results.