oh, and one other bit of advice of potential use is for you to get one of the Railcards aimed at young people over 16.
Also - assume you gave them your correct name and address to write to you about this - as if not it will cause probs for you as it will likely end up in court without you knowing and take a lot more to untangle. Good chance that GWR will settle without court if you engage with them as per
@Hadders sound advice.
It could take a while for you to hear. If you are going to University in sept / oct I presume you will have someone at the address you gave to monitor your post (it may take in theory up to 6 months before you hear from them), if not you will need to pay Royal Mail to redirect any post to you. Chances are you will hear much sooner than that however and be able to deal with it before too long.
You can get more helpful advice from this forum as the process goes forward. You will also need to budget for the payment you will need to make to resolve this. Other examples are on the thread but that may be in region of eg £200.
Your key objective is to try and seek a settlement that means you do not get taken to court and receive a criminal record - which is not really what you want in life and especially not at your age with uni and the future stretching out ahead of you.
But also try not to worry too much as there are ways to resolve this all being well.
Good luck as I am sure you have already learned from the mistake you made.