Pokestops are special designated places in the real world that you can use to attract pokemon, catch them, and get cool items.
The best way to find them is to take a walk! On the screen you'll see them as big blue signs on the map. After you use them they turn purple, indicating a wait time before you can use them again.
The writers at Tom's Guide wrote a little How-To on how to be the very best, you can find it here:
https/www.tomsguide.com/us/pokemon-go-guide,review-3761.html