Heart Rate. Take your pulse for 6 seconds, then multiply that by 10. This is your resting heart rate. Do this several times to get an average number.
To determine your maximum heart rate subtract your age from 220. DO NOT exercise at this rate!!
To determine the rate at which you should train multiply your maximum heart rate by 80%. For example, I'm 38, so my training rate is calculated: Maximum rate is 220 - 38 = 182. Training rate is 182 x .80 = 145.
Never exceed 85% of your maximum heart rate when exercising.

Now let's use a little JavaScript to calculate your max heart rate and your training rate:

Source: Fit or Fat? by Covert Bailey, 1978