Get yourself examined and rule out hormonal disorders such as gynaecomastia or male breasts. See an endocrinologist.
But, fat deposition on chest and abdominal area is common due to weight gain. Avoid fatty foods, fried foods, desserts, sweets, chocolates, desserts etc and follow a strict diet of protein and whole grain carbohydrates. Start cardio exercises regularly in the form of running, jogging, cycling, swimming etc to tone down and have a proper body weight for your height and to come to a BMI less than 25.
You can't just spot reduce areas of your body, you need to lose weight overall and then the areas that you store excess fat (belly etc) will diminish accordingly. So, a healthy diet and lots of interval training is the main thing. Then you can make the "man boob" problem less noticeable, by firming up and toning your upper chest area with a decent weights routine and push-ups. Sometimes it's a hormonal issue though (gynecomastia) and surgery or Gynexin pills would be the next step...