Determine the gender. Nouns in Italian are either masculine or feminine and demonstrate gender with the word ending. Usually, nouns ending in "o" are masculine (singular), "i" (plural), while nouns ending in "a" are feminine (singular) "e" (plural). There are exceptions and irregular noun endings, but let's focus on the basic forms first:
ragazzo--boy; ragazza--girl
ragazzi--boys; ragazze--girls
In the above case, gender is apparent. However, for inanimate objects, gender also exists: libri--books (libro, masculine singular); case--houses (casa, feminine singular).
Masculine plural nouns ending in "i" generally have a singular form ending in "o":
graffiti (pl.); graffito (sing.) words written or painted onto a wall
paparazzi (pl.); paparazzo (sing.) intrusive newspaper reporter
carri (pl.); carro (sing.) car, carriage
Replace the final "i" with an "o," and you have the singular form of the noun.
Feminine plural nouns ending in "e" generally have a singular form ending in "a":
penne (pl.); penna (sing.) pen (penne pasta? Pasta shaped like pens.)
pizze (pl.); pizza (sing.) large round dough base with sauce and cheese
scrivanie (pl.); scrivania (sing.) desk
We never see the plural form, pizze, in English, as we have made the word part of our lexicon--so we say and write pizzas.
There are many irregular noun endings that simply have to be learned in order to recognize whether they are singular or plural (or even masculine or feminine). That's why it is recommended that you use a dictionary to check the forms. For example, the word for a university, "università" is the same whether it is singular or plural:
l'università--the university; le università--the universities
la città--the city; le città--the cities
il tassì--the taxi; i tassì--the taxis
il caffè--the coffee; i caffè--the coffees;