I'm apparently a natural high-cadence guy--the first time I got on a real bike, more than 40 years ago in college, I immediately cranked along at 90-100. I didn't find out that was a good thing for a year or so. Since i've never had to think about it, my advice may not be the best available, but what occurs to me is, "Why don't you just go ride?" When you find yourself grunting along at 40 rpm, shift down and pedal faster. It's worth working on, but it's not very complicated.