How Do I Lose The Fat On My Chest And Gain Muscle On My Chest And Abs?

I used to be fat, but I lost all the weight and now im pretty skinny, but theres fat left over on my chest, and I want to get rid of it and gain some muscle there, and on my abs, but I dont know how.
Im 5´5, 124 pounds( I used to be 149), and im 15 years old.

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