Welcome to the 'Board! A couple of good responses already. I'll add a few things.
Get as much experience in as many different environments as you can. Get in your local quarries, the ocean, do shore dives, boat dives, travel, learn your local dive sites. Do night dives, deep dives, learn to navigate so you're never lost, get nitrox trained and certified, learn proper gas management. The more you know and have experienced with diving, the better divemaster you'll be.
As for the course, it's a lot of book knowledge. You'll need to know physics and physiology (learn the Encyclopedia of Recreational Diving). You'll also have to be able to perform all the skills you did in your OW class (and a few more) presentation quality.
Start reading the encyclopedia now and getting your experience in a variety of sites. If you dive a lot, you could be ready in a year. I was, but I also had about 130 dives my first year and in almost a dozen different types of dive sites - local lakes, So Cal boat and shore diving, Sea of Cortez, NE quarries, NJ wrecks, Florida springs, a missile silo, the Caribbean, just to name a few.