What should your maximum heart rate be while exercizing?

In general your maximum heart rate while exercising should be 85% of your predicted maximum heart rate. You calculate the maximum predicted heart rate by subtracting your age from 220. In your case it would be 220-39=181. Taking 85% of 181 gives a heart rate of 153.

If you are not in good condition, you may not be able to exercise the level to achieve this heart rate initially. Another way to predict your target heart rate during exercise is to start with your resting heart rate. Then exercise and see what the maximum heart rate you achieve is. Tkae the difference between the maximum heart rate and your resting heart rate. Then take 1/2 of this difference and 3/4 of this difference and add these numbers to your resting heart rate. This will give you an initial target heart rate range while you begin to exercise.

I WOULD SAY AT 120-140 BEATS A MIN , REMEMBER GIVE A LIL FOR YOU WEIGHT