While I agree with a lot of the post here regarding experience - at the same time I appreciate your enthusiasm and determination to get this done! Our industry needs more professionals with your passion and a lot of us tend to forget we were all green once too! There is a lot of good advice here and I agree with some of it. You definitely need a lot of background in physics, physiology, dive procedures and safety. FAR MORE than you will probably get in your course depending on the student pool you will be teaching in. The more background you have the better instructor you will be.
However, one of the pieces of advice I would add to most of the above is SWIM. As an instructor trainer who has literally trained hundreds of instructors, one of the most common issues I see in the water is candidates who cannot focus on the material presented pool side or in open water because they are overtaxed physically. You should swim 1000 meters in mask, fins and snorkel and 500 meters free swimming 3 or 4 times a week if possible. Test yourself - you should be able to complete an 800 meter snorkel swim in less than 14 minutes (comfortably) and a 500m free swim in less than 12 minutes comfortably. Also practice swimming 25 m or farther underwater on one breath. Then grab your gear - an instructor candidate should be able to hover motionless for at least 2 minutes at a set depth (+-1 ft) in a normal face down swimming position. You should also have a superior skill level for all other basic dive skills.
DIVE EVERY CHANCE YOU GET until your course begins (and of course during it as well)
Aside from that read everything you can find both from your course materials and other materials. Review all of the diving accidents you can find and analyze the causes of the accidents - learn from the mistakes of others!
Good luck with your class and even though I am not an advocate of the xday wonder classes - if you are committed you can come out of the course as a pretty good instructor. So do not let anyone rain on your parade - keep your enthusiasm - and enjoy your program. Finally, never accept short cuts not from yourself and not from your instructors - do not cheat yourself and do not let anyone else cheat you out of effective training!
Safe Diving,