If you've just turned 15, you can be sure that your body is not done developing yet and you will see some more changes. However, we can't really predict how much more your breasts will grow, and there's really no influencing it.
Edit: Joey got to it first But I'll leave my reply because there are some more links I think you should look at
Hi ChocolateBubbles, welcome to scarleteen! There is no way to "make" your breasts keep growing, I'm afraid. It's also hard to know if your breasts have completely finished growing; puberty differs for every person, so it may be that for you personally, your breasts have stopped growing, or they haven't. Puberty actually lasts into one's early twenties so you're certainly not finished, so it may be the case that your breasts will continue to grow.
However, plenty of people have A cups in adulthood and it's nothing to be ashamed of or feel bad about Everyone is different -- you are the only you in the world! Learning to celebrate and love that would be a great mindset with which to approach life
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