If you are overweight,it's very important to loose your weight through a healthy balanced diet and exercise(mainly cardio).cardio exercises are exercise which generally increase your heart rate,improve your stamina etc. and are beneficial for heart.cardio exercise could be anything like a brisk walk for 30 min on an even surface like a park or walkway or can do swimming or treadmill,bike,cross trainer,elliptical etc,but since you already have a heart problem better would be to exercise under a trained professional.