The Italian indefinite article (l'articolo indeterminativo) corresponds to English a/an and is used with singular nouns. It also corresponds to the number one.

INDEFINITE ARTICLES
MASCHILE	FEMMINILE
uno zio (uncle)	una zia (aunt)
un cugino (cousin, m.)	una cugina (cousin, f.)
un amico (friend, m.)	un’amica (friend, f.)

Uno is used for masculine words beginning with a special consonant like z, ps, gn, y or s + consonant;

un is used for all other masculine words.

Una is used for feminine words beginning with a consonant;