20mins of cardio activity (cycling, swimming, running, walking, elliptical training), at least 3 times a week.
during this 20minutes of activity, you will have to measure your heart rate periodically (every few minutes) to make sure you get it up and maintain it at the target level.
if you want to have a more "healthy heart", aim for the green zone level in this chart. it depends on how old you are, but if you're in your 20's, your heart rate should be 100-120 beats per minute to be in the "healthy heart" zone.
if you want to start burning fat and losing weight, you have to increase the intensity of your activity, until you reach the "weight management" green/yellow zone of 120-130 beats per minute.

to measure your heart rate, you can place your fingers over your carotid artery (just to the side of your Adam's apple), or you can wear a heart rate monitor.