First of all welcome to the forum!
I'm glad you are getting into horse judging! I did FFA and 4-H judging contest when I was in high school. It's really a great experience.
What kind of resources does your coach already have? We had an entire library of practice judging videos that we used to get ready for a competition. They were a similar set up to this:
It shows you the class, giving you time to take notes and decide how to place the class, and then afterwards will tell you how the class was actually placed and why. This is just a sample video so it doesn't show the whole thing, but its well made. If you don't have any this is a good company to go with. Our regional contests were all held at MTSU. Dr. Whitaker is really good.
But the tapes are the first thing I would invest in if you have the resources to do so. I think you'll get a lot more out of them than you will looking at pictures especially when it comes to the performance classes. Also if you have any good show barns in your area you can have your leader contact them and ask if they would be willing to give a live demonstration.
Judging the class correctly is only part of it though. You will also be required to give oral reasons to defend your placing of certain classes. Be sure you take notes while you are judging to help you remember the class and write your reasons later. AQHA has an excellent manual for reasons that I believe they will send to your group for free if you have your coach contact them. If not you can download it as a pdf online here: Test Your Judging Knowledge – America’s Horse Daily
I hope that helps. If you have any questions please don't hesitate to ask.