firstly you are going to need to loose some fat, to work out the calorie intake to loose weight first work out what your
resting metabolic rate is, you need to eat less calories than this number and do some exercize, you can't spot reduce the fat but you can build muscle which will help with loosing more fat and make you look better.
to tighten your man boobs and replace with muscle you are going to need to work your pec muscles (Pectoralis Major (Sternal Head)) here is a good list of exercizes to work that area
http://www.exrx.net/Lists/ExList/Che...l#anchor165455 but compound exercizes with free weights are generally better and work more muscles.
i am sure some of the other here will have more great sugestions and give you all lots of support in your fat loss goals.