From the American Heart Association:
One way to gauge your heart rate during exercise is called "conversational pace"---if you can talk and walk (or your preferred exercise), you're not working too hard. If you can sing and maintain your current level of effort, you're not working hard enough. If you can't catch your breath, you are working too hard!
If you participate in more-vigorous activities like brisk walking and jogging, the "conversational pace" approach may not work. Then try using the target heart rate. It works for many people, and it's a good way for health professionals to monitor your progress. Subtract your current age from 220. This is your maximum heart rate. For instance, mine would be 220-43=177. Target heart rate should be 50-85% of your maximum heart rate for optimal cardiac health. To make it even easier, you can see the link I've attached. The AHA cautions that if you're not exercising, you should begin at or even below the 50% goal and then progress higher as you become accustomed to exercise.