Have a look at the Training thread where you'll find stacks of threads on how people are using their HRM in training.
Basic formula for working out your MHR:
205-(your age/2). But, and it's a big but, practically everyone I've read about on the forum seems to find their actual max is different, usually higher. So get the book, then as soon as you can do a stress test (the book explains how) to find out your personal max heart rate. Without that, the HRM is potentially very misleading as a training tool.