First of all, i would like to reassure you that what you have noticed is quite common among boys of your age. It is called Gynecomastia
, and can be self limiting. What that means is that most of the time, it will get better by itself, if you give it a few months. There might be a period when both breasts may enlarge as well. Going to the Gym or dieting will not help. Only rarely, if it has not disappeared after one year, or if it is painful or if it is rapidly growing, they need surgery but 80 % of the time, they will get better without surgery.
Dont be anxious. wear loose clothing and give it time.