Resting heart rate v. target heart rate

Two fingers on your wrist (don't use your thumb). Find the midpoint on the inside of your wrist and aim slightly to the left and you should feel your pulse clearly. Count the pulse for 60 seconds when you're in a calm and steady state and that will give you your resting heart rate.

You then need to work out your maximum heart rate. That's just 220-your age.

The general idea is that working out at 60-70% of your maximum heart rate will put you in a fat burning zone and getting closer to 80% will put you in a cardio zone (so improving your cardio performance). I can't say I've noticed a lot more than the harder I work out, the more calories I burn and (provided I don't subsequently overeat) the more weight I lose correspondingly.

Statistically, your maximum heart rate is 220-age. However, this equation is not correct for most people. Besides, there's no heart rate for weight loss. Keep in mind that weight loss is done in the kitchen and sport is for being healthy and fit. That's all. If you want to run, swim or do any other cardio then chose a variety of speeds, for example:
long and slow, so slow that you can still easily talk or sing to music. With long I mean long. Not just 30 minutes but as long as you can with that slow speed. Try to increase distance each time.
Slow and fast. Chose a reasonable speed and try to go as long as you can. Try again next time. Up speed regularly. Or do intervals.
Something in-between both. Make sure you can still talk, but not as easily as in the long and slow day.