Your breast grow and you start your period if your female oh and your voice changes. You start becoming more mature. Guys start growing down there also and more voice change than females. I'm trying my best to keep this answer clean:)
Well first of all, for boys and girls they are obviously different. For boys most notable ones might be beard starting to grow, voice changing, shoulders growing bigger, stuff like that... and for girls breasts starting to grow and their period starting.