For a healthy individual with no history of cardiac disease the easiest way to calculate your target heart rate is by first estimating your maximum heart rate. A rough approximation of your maximum heart rate can be found by subtracting your age from 220. Therefore, if you’re 30 years old, a good estimate of your maximum heart rate would be 220-30=190 beats per minute.
If your goal is to decrease your risk of developing coronary disease, then you should exercise between 50% and 60%your maximum heart rate consistently for 30 minutes, 6 or 7 days every week. If you do not need to lose weight, low intensity activities such as walking may be all you need.
If your goal is to reduce body fat you should train at a level of 60%-70% of your maximum heart rate, and if your goal is aerobic conditioning and weight management you should train within a zone of 70%-80% of your maximum heart rate. For a detailed and more accurate method, I have provided a link to a target heart rate calculator which also accounts for gender and resting heart rate.