In Italian, unlike Spanish, the indefinite article exists only in singular, if it is needed in plural (a man crosses the street / some men cross the street) it must be replaced with an indefinite pronoun. Since we have to talk about indefinite pronouns later, we will not talk about the plural right now.
The whole picture is similar to the one we have already seen in the case of the definite articles.
masculine singular if the noun doesn' t beginn with a consonant but not with s + consonant, ps, z, y, gn, ps, y
masculine, singular if the noun begins with s + consonant, ps, z, y, gn, ps or x
feminine, singular if the noun beginns with a consonant
feminine, singular if the noun beginns with a vowel
It is important to see, that there is a big difference in feminine and masculine nouns.
1) In general the indefinite article for masculine / singular is un, for feminine / singular una.
2) Only in the feminine form wie have an apostrophe in front of a vowel un' (un' amica / a friend)
3) Ony in the masculine form there is a change before s + consonant, ps, z, y, gn, y