My sister threw my phone and the screen completely fell off.

This is an inexpensive phone so the parts aren't that expensive first unhook the battery. Did the cable for the screen tear? If the screen is ok and the cable is ok hook up the screen and connect the battery and see if it works. If it does you will have to buy some double sided tape for cellphone repair. You can purchase some from Amazon. They have different widths and you will need to cover the areas were the frame contacts the screen. If the screen or cable is damaged you will need to purchase a new screen, you should be able to find it from ifixit and follow their repair videos. You can also find more repair videos on YouTube.