To calculate for your Maximum Heart Rate, you must subtract your age from 220. 220 is the value of the maximum heart rate that humans can actually produce in a lifetime. From the examples below:
220 - 20 years old = 200 beats per minute (Maximum Heart Rate)
This means, that if you are 20 years old, your maximum heart rate should be at 200 beats per minute only.
Target Heart Rate Zone pertains to the percentage of increments that are added in exercises. This actually depends on the specific goals you want to reach for yourself.
If you want to build endurance and lose weight your target heart rate zone should be between 60-70% of your MHR. 70-80% you are now targeting cardiovascular fitness and weight management. Anything higher than 80% is a good indicator for interval workouts.