Girls
You grow taller. Your shoulders and hips may broaden. You could put on weight for a while. Your breasts grow. You grow hair under your arms, pubic hair around your vulva, and you may notice more hair on your arms and legs or around your nipples. Your voice gets a bit deeper, though not as noticeably as a boy's does. You hair and skin might get oily and you might get pimples. They won't last forever.
You may already have started having periods. Blood comes out of your vagina for a few days, usually about once a month. It means you are now able to conceive - in other words, male spermatozoa can make you pregnant.
Boys
You grow taller - and stronger. You grow hair under your arms, and pubic hair around your genitals, you sprout whiskers and maybe a hairy chest. Your voice gets deeper - sometimes overnight. Your hair and skin might get oily. You might sweat more and develop your own body smell. You might get pimples. They won't last forever.