I had manboobs, hell. I was 210 lbs and I'm only 5'7. Now I've lost my manboobs....and in pretty decent shape.
The most effective way is proper diet, you can do some cardio. Like some people said, do chest workout. HOWEVER! don't do too much decline. Not yet anyways, do more incline so you can lift your chest up higher. the only way to lose it is to drop your over all bf%. There are no shortcuts, no pills will help you. They will dehydrate you and possibly kill you. Don't stop eating, a lot of people mistaken diet means eat one meal a day and starve yourself. NO! you want to have more smaller portions and increase your body's metabolism, if you do cardio and weights, it'll accelerate the process and makes you look better.