I am 6'4" and around 250 lbs and i want to lose weight. how i can decrease the size of my chest (man boobs) and stomach?

Calories in n out: See the excellent answers on "weight loss". You basically need to burn more calories than you take in. Increase your exercise, choose healthier, less-caloric foods and drinks. The nice thing is that if u build more muscle you will even burn more calories while you sleep! and don't focus too much on weight. Focus more on pants-size, etc. Muscle weighs more than fat but that's ok--its good wt.

Lose Weight First...: You are wise in achieving a long-term stable weight. You will find that a regular exercise program and a healthy diet will go a long way in helping you achieve your goals. Once you have achieved your long-term stable weight, if the gynecomastia is persistent, surgery may be indicated. It is often best treated with partial excision of the prominent glandular tissue and peripheral liposuction.