Congratulations on the offer of employment!
As you've probably read on this forum, you've done incredibly well to get where you are. The training staff know this and will do everything they can to train you to the required standard in rules, before you head out to your depot for the practical work.
When you're classroom based, the most important thing I'd say is, if you're not sure, ask questions. It may be that theres a different explanation that clicks better with you. Me, I'm a visual learner so you can tell me about it till the cows come home n I won't understand, I need to see it. Don't be afraid to stick post its in your rulebook, highlighting important bits or explaining things clearer for you.
The career change is daunting, though a retail background will give you the mental agility when dealing with grumpy customers to keep them onside.
Above all, enjoy the experience.