This usually happens after you start to develop breasts, but for many girls, pubic hair starts first. You will probably get pubic hair before you get underarm hair. Underarm hair usually comes near the end of puberty. At first, you will probably just have a few fine hairs in your pubic area and under your arms. Late in puberty, the hair will become thicker and curlier.
Some girls decide to shave the hair under their arms.
There are no health reasons to do so, but some girls simply prefer not to have underarm hair. It is up to you if you want to shave. Talk to your mom or another adult that you feel comfortable talking to about this. Most girls have a growth spurt the year before they get their menstrual period. Your feet and hands will usually be the first parts to grow, and then the rest of your body will follow.