Hi Sam, it's not too bad, remember everyone in the class will be in the same boat as you! I felt uneasy going to my first class but the tutor made us all feel at ease.
Everyone will be asked to share their DD experience so you will probably find there are actually people in worse situations than you there anyway.
They will cover alcohol units and how quickly alcohol leaves your system which is really useful to know. Also you'll learn about the effects of alcohol on your body - not exactly riveting stuff but still it is good to be informed.
They will let you know about insurance companies who will give you discounts for having completed the course too.
My course was over 3 saturdays, it is the same for you? It goes quickly and then you will have a shorter ban - definitely worth it!
One other thing, try not to be late, my tutor would mark people as absent if they were more than 20 mins late!!
Good luck x