How do I start working out at 50 and overweight?

You can start with low intensity exercises such as walking for a period of time three or four times a week. Starting with low intensity exercise to allow your body to gain fitness and strength, then increasing the intensity from there. Other forms of low intensity exercise like cycling or swimming will be good alternative to walking, and will provide the same benefit to aid you in your weight loss journey. 

Resistance exercise with small weights can also be done to strengthen your arms and upper body, while providing weight loss benefits. Coupled with a healthy diet and enough rest, your body will respond well to exercise and your weight will come down. Keeping the heart rate low during exercise will limit any chances of adverse symptoms arising while exercising. Should any adverse symptoms arise, consult with a medical professional immediately.