Below are the instances when we have to use "ser"

with an adjective to express a permanent or inherent quality
El azúcar es dulce

sugar is sweet

mi padre es calvo my father is bald
mis hermanas son altas my sisters are tall

for occupations and nationality
Son italianos

they are Italian

Pablo es español Pablo is Spanish
Es profesor She is a teacher

with nouns or pronouns
es un coche

it is a car

son niñas they are girls
somos nosotros it's us
¿Eres John? Are you John?

with possessions
es mio

it is mine

son de Pedro they are Pedro's
este es mi libro this is my book

with time, dates and numbers
son las seis

it is 6 o'clock

dos y tres son cinco two and three are five
Es el 21 de diciembre it is 21st December
la temperatura es de 20 grados it is 20 degrees

with events

when there is an event or an occasion such as a party we use "ser" even though we may seem to be indicating position which is usually with "estar"

La fiesta es en casa de Susana The party is in Susana's house

with passive constructions

Although it is not as common as in English there is a Spanish equivalent of the English passive.

fue construido en 1800 it was built in 1800
el cuadro fue robado The picture was stolen

when speaking about what something is made of
esos zapatos son de plástico those shoes are (made of) plastic
es de oro it's made of gold

with "how much / many?"
¿cuánto es? How much is it? (asking about the price)
¿cuántos son? How many are there?