As someone pointed out recently, exercising is not what you should focus on to lose weight. Most people don’t lose weight by exercising. In fact, some people gain weight because of the muscle (which is heavier than fat) build up.
So what should exercise be for? Well, the most important reason will be to strengthen your heart and lungs. It has been proven that an aerobically fit individual can work longer, more vigorously and achieve a quicker recovery.
How often should I train? How hard? For how long?
Most experts believe that 3-5 times per week for a duration of 20-60 minutes.
WORKOUT INTENSITY
How do I determine my target heart rate?
The general formula for the average person is 220 age X 60% and X 90% of HRmax. For example, a 30 year old would calculate his target zone using the above formula: 220-30=190. 190x.60=114 and 190x.90=171. This individual would try to keep his heart rate between 114 (low end) and 171 (high end) beats per minute.
Calculate target heart rate by subtracting your age from 220
(220-30=190).
Subtract your average resting heart rate from target heart rate
(190-70=120).
My friend uses a Polar Watch with an attachment to his chest that continually monitors his heart rate. Its expensive, but for someone serious about training, would need it. For most of us, just exercising is really good enough. And when you get into the habit of it, then consider buying the gadgets because then they won’t go to waste collecting dust 
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