Welcome to the pond & I hope you enjoy your course & your new hobby. I remember being beside myself with excitement and wondering what my 1st scuba class was going to be like. I was lucky in that I was given a 400+ page book for the course (Sukellus by Vikman, fabulous book, very unfortunate that it's never been translated). Being a bit of a geek, I read most of it: a huge relief for a student desperately wanting more info & something to do for preparation.
Look on the Internet esp. for blogs by persons taking different classes. I found a great, detailed and fun blog once by someone taking the GUI course - don't know what the link was but maybe you will happen onto it as well.
Here's a few items I found for you:
Scuba Diver Open Water Diving Adventure Courses - PADI Scuba Diving Training Organization
The Daily Apple: Apple #469: Your First SCUBA Lesson
What to Expect on Your First Scuba Dive - Learn to Scuba Dive
Look too at the websites for diving magazines. They run "1st scuba class" aticles fairly often and some will be online in full to read for free. For example
How To Scuba Dive | Scuba Diving Training & Certification | Scuba Diving
Training - Divernet
IMHO if you want to prepare, it might be helpful to spend some time snorkeling. We seem to get a lot of adults now that clearly have never worn fins before, some of whom have never had a mask on & their face under water. It's always better to have a student who feels at home in the water and using the basic equipment (basic equipment meaning mask, snorkel & fins).
The one thing I would say is that if you have trouble with some skills as you are training, don't think that means you don't have the right stuff to be a diver. LOTS of people have trouble with various points, that's why we train and take it slow and easy. I've noticed some of the 5+ tech cert instructors on this board commenting how they has trouble with for example, with the mask clearing/no mask drills as a basic open water student. It's a long learning curve, that's part of what is great about scuba - there's always more to learn, more to practise, new skills to try.
Hope you have a great time!