You use Ser to describe someone’s/somethings identity or to talk about time and events, and estar to describe someone’s/something’s state or something’s/someone’s location.
ex: está muy triste hoy (she is very sad today)
nuestro coche está roto (our car is broken)
estoy en la escuela (im in the school)
ex: soy cantante (i’m a singer)
Tú eres una niña (you are a girl)
somos inteligentes (we are intelligent)
down below btw is a chart