Don't worry or panic.
Just go in, go up to the front counter and say your there for a welcome meeting. It will depend on the store, but for us, we hold welcome meetings in the Crew Room, someone will take you there.
They will show you a video, about when McDonald's opened, and blah blah blah - you just have to watch it.... you don't have to give a sh*t about anything that video says - all it is there for is so the person taking the meeting has some time to go and photocopy your passport and national insurance card.
If they have previously asked you for your clothes size, they will give you your uniform which is usually 2 'designer' McDonald's T-Shirts, 2 Pairs of McDonald's Trousers with no pockets so you can't steal, a McDonald's Cap - with you will have to guard with your life, an Apron (Hopefully you'll get the new Deli Choices aprons - They are really nice!) and your name badge :D
- Odds are it'll be the wrong size so if you want, you should ask if you can try it on before you leave. (There is often a changing room for staff)
You will also be given a folder with your temporary employee card... keep it safe, because despite working there for over 2 years and being a Crew Trainer, I still don't have a proper card. :)
You will then be asked to go home, and get a Health and Safety certificate online, bring it into the store, and you'll then be booked in for your first shift.
Good Luck! :D