Welcome to Android Central!
Open the app drawer (either by swiping up from the bottom of the screen or by tapping on an icon that looks like a grid of dots). Scroll through the list until you find Facebook. Press and hold it, then drag it to your homescreen.
If you don't find Facebook among the apps in your app drawer, then you'll need to open the Play Store app and install it from there.
If you need additional help or have more questions or details to share, please join the site so you can reply in this thread. See
this link for instructions on how to join Android Central.