Agar.io game is a simple game where you basically eat others and get bigger. You are a small ball at start and you move your mouse around the screen to control your ball. At first try to avoid everyone and just try growing bigger by getting some computer generated balls. Once you are bigger, try finding bigger balls that move around, just make sure they are not bigger than you, because then they would eat you instead. Grow as big as you can get without dying and repeat.