I am over weight. what is the best exercise for me. my heart rate on average is 120.

Aquatic exercise: Swim, water aerobics or walking in chest deep water. We are about 80% lighter standing in chest deep water due to buoyancy. This will make exercising easier. Start with an amount that is relatively easy and comfortable, then repeat 4 or more days per week and add a few minutes to the program once a week, initially only add 2-3 mins per week, so you don't over do it. Don't give up and keep moving.