Target different areas of your pectoral muscles with a mix of various exercises. General for your "upper" chest you're going to want to go for lifts that involve vertical movements from an angled position (Barbell/Dumbbell Inclines, Elevated Pushups, Bench Press, etc). For your "lower" chest generally you're going to want to do horizontal movements as well as some angled positions opposite of what you do for your "upper" chest (Barbell/Dumbbell Decline, Dumbbell/Machine Flys, Dips, etc).
In addition, you need to consider your overall dietary needs. If you're higher in body fat, the surest way to have more defined muscles is, of course, to lose body fat. However if you're trying to build up muscle, you can't try to "build" (Being in an anabolic state) while simultaneously trying to "lose" (Being in a catabolic state). Personally it's easier to build up muscle first and then cut the weight, but you can try to lose excess weight first and go from there.
Regardless, your macros should be filled mostly with lean meats, low GI carbs such as vegetables and moderate whole grains, some fats to keep you full. If you can get at a minimum of .5 grams of protein for every pound you weigh, you're in a good spot. If you can match 1 for 1, that's best but difficult (And perennially dangerous) to do long term.