Signs that your period is on its way are if you've grown underarm and pubic hair. Typically, you'll start your periods about two years after your breasts start growing and about a year after getting a white vaginal discharge. The average girl will get her first period around 12 years old, but it varies from person to person.
Here is a graphic of the muscles used in push ups. Compare it with the following about bench press. Both will give you a bigger chest and some more arm and shoulder size. Push ups work the stomach more. I've been working out in the gym for more than 40 years. My advice to anyone who will listen is to do a lot of push ups. Yes, I know the bench will get you stronger than push ups, but it is push ups that will allow you to bench when you get my age.
These are the 2 large chest muscles and mainly the pushing muscle group of the upper body. As you are pushing and lowering your body during a push up, the pectoralis major is doing most of the work. As a result, these 2 muscles become very strong and can become defined lean muscle after doing push-ups regularly.