To lose fat from anywhere on the body you have to have a good diet and do plenty of cardio. Cardio training can be done in a variety of ways, running, bike, rowing machine, cross trainer, just nto name a few. The cardio should last 30 minutes to 45 minutes minimum and be done several times per week. Once you are coping with that add in some interval training as part of your cardio training.
When you lose bodyfat due to training the fat is taken from all over the body, so remember there is no such thing as 'spot reduction' meaning that even if you do a hundred or a thousand push ups a day you may still not lose the fat on your chest. Therefore choosing a cardio exrecise that works as much of the body at once is helpful, I find a rowing machine is one of the best.
However if you have previously had a hormone imbalance (there can be a number of reasons for having an imbalance) than reducing the appearence of "man boobs" may require surgery.