How to get a sexy and round boobs shape naturally without bra?

Genetics. Inherit great genes from your parents. Otherwise eat well, exercise and stay lean and healthy -- good luck, there's no instruction manual for this one.
Be born to a mom... The only way to get naturally round "boobs" is to be born to a mother with such "boobs" and to have aunts with similar roundness. With those genetics, on can live a healthy life, get plenty of upper body exercise, eat nutritious natural foods, not be too thin nor too overweight, and the roundness will just happen naturally.