I agree with congratulations especially in a competitive market. You must have done well in application and interview process.
Procedures seem to be revealed gradually rather than issued in one fat formal handbook. Shift rotas (each standard shift is four hours a week for someone your age) are organised and communicated online and may change to ensure cover so don't get muddled up or forget, as you are on probation for the first three months.
Make sure you use this time to shine and don't put a foot wrong. Keep up the work and image / attitude re pleasant helpful and keen ALL the time! It may look casual but there is an expectation that you greet and help and look keen and enthiusiastic through out your shift no matter how many times you have said ' hi' at the door! or been up and down stairs and smiled and folded or shown people to the changing rooms!
75% off your outfit for uniform.. taken from a look in a current season. Certain number of looks allocated to each store for staff to wear so first come first served as to what you end up wearing for a term. You will learn about discounts for you and named two or three personal contacts. Discounts are for main items not high end cost garments and some are banned for discount.
But all this you will pick up in your first few weeks especially if the store is not a new one and there are staff who are already settled in. Different in a brand new store with all new staff team.
I wish you well. It is exciting but remember that as with lots of these stores where there are so many who would love to work there, the three month stage will be a day of reckoning!!
Have fun !