Just plug in your max heart rate and resting heart rate, and it gives you the ranges for the 5 training zones. MHR is genetically predetermined and lowers by 1 beat a minute as you age--not much you can do about this number; but resting heart beat indicates fitness level and lowers as you get fitter. This marathon guide suggests a few ways to determine MHR, including running some uphills. A monitor is probably the only way to get an accurate number. Otherwise, they tweek the formula, suggesting: 214 - (.8 x age).
The link to the training zone article gives some good information on this whole approach to training and what to look for if in the market for a monitor. I'm getting close. Taking my pulse with my finger isn't cutting it.