Take a couple of slow deep breaths. Thats the quickest way.
If you can, go somewhere quiet. Sit down, close your eyes and relax. Focus on controlling your breathing. Breathe in slowly til your lungs are full, hold it for a few seconds, then let it out slowly. Repeat a couple times.
If you are talking about chronically having a fast heart beat, known medically as tachycardia
, then you need to talk to your doctor, he or she may put you on medication, or may just tell you to avoid stimulants such as caffeine, cigarettes, energy drinks, etc and exercise more.