If you do not know your maximum heart rate, it is easy to estimate based on your age using the equation: Max HR =  220 bpm - your age. 

Maximum heart rate will vary from person to person, but often goes down in relation to your age. Using this base line, you are able to accurately estimate your HR and change it as needed if you see a higher HR or consistently see a lower rate. You will likely only see your maximum heart rate in sprint-type efforts, so it may take a few classes wearing a heart rate monitor to verify if your value is correct.