To find your maximum heart rate, take your age and subtract it from 220.
Maximum Heart Rate = 220 - Age
It is recommended that one should work out at 55 - 90% of maximum heart rate. So, to figure out what intensity is right for you, chose what percentage of intensity you should work out at and calculate your target heart rate during exercise from your maximum heart rate.
Target Heart Rate = Maximum Heart Rate X ( 55% - 90%)
Beginners should work at the low end of this recommendation and increase intensity as appropriate. Thus a good target heart rate for a beginner should be 60% of the maximum heart rate.
Thus for a 45 year old person just starting an exercise program the appropriate target heart rate during exercise should be.
220 - 45(age) = 175 Maximum Heart Rate
175 X 60% = 105 Target Heart Rate During Exercise
Remember, however, to consult your physician before starting an exercise program.