Study up on gynecomastia.
There's a number of things that cause gyno. Only about 1/3 of the population has the gene for it, and contrary to wrongful belief, not everyone has the receptors to get gyno. It's caused by estrogen receptors in your chest. Sources of estrogen could be yourself (all testosterone breaks down to DHT and estrogen). Both estrogen (the female sex hormone) and testosterone (the male sex hormone) are steroids. Steroids are those hormones soluble in lipids. Cortisone is another steroid. Common sources of estrogen include food, exogenous testosterone, prescription drugs, soy, pot. Your indigenous testosterone automatizes to estrogen which will cause gyno, as it comes up until you get around 28, and it peaks. It's easily treatable with a SERM, or estrogen receptor antagonist, available online, or through a doctor. Some folks naturally have gyno. The gene is present in about 1/3 of the population, and some folks will get it no matter what their behavior. You can have them surgically removed (the mammary glands) and you won't have the problem anymore.
It's not clear in your posting whether gyno is your question, or if you just don't like the shape of your chest. If your issue is the latter, know that there's not a lot of ways that you can change how your chest lays as it develops. For development, incline dumbbell presses are the most effective. Flat bench works your shoulders, triceps, much more, and, if you don't do posterior deltoids exercises shoulder impingement is a given. Flat bench with a barbell is especially bad on your shoulder girdle.
About the only way you can change the shape of your chest is by expanding the fascia in certain places and not in others. That's commonly done by spot injections in the spots you want to enlarge. You enlarge the casing, and the muscle grows into it. The fascia is a limiting factor in muscle growth, and, if you manually expand it, the muscle will grow into it. Slang for spot injection is Pump and Pose. The original Pump and Pose uses stuff I sure wouldn't put in my body. If you choose that path, a better choice is MCT oil with 4% BA.
A note for the ignorant here, raised by MsclDrew's comment about fat distribution: Anyone using elevated levels of testosterone will have a much more even fat distribution than someone who doesn't. He spoke about his uneven fat distribution which is indicative of someone who doesn't have high levels of androgen in their system.