At fifteen, your body has not yet finished its development, and it is not unusual for breasts to grow at differing rates - they even out with full maturity. Havew a word with your local lingerie shop, and have a proper bra fitting, they can advise, and if you are still worried, have a chat with your GP.
And no, creams will not work. As AH says, see a gp if you are worried, but it is very normal.
Also many women have uneven breasts.....I cant claim to have had experience of a representative sample, but Ive yet to...urm...meet a lady with a completely even pair. Human bodies are not generally symmetrical. Ok we all have two of lots of things but they are generally slightly diffeent sizes. Dont worry, things will even up.