How do you form the possessive in Italian?

The possessive in Italian depends on the number and gender of the object, not the subject. 

For example, 

"Il suo gatto" / "His/her cat" (nm)

"Le sue scarpe" / "His/her shoes" (nfpl)

In Italian the possessive also uses the definite article, so it is formed as follows: 

io: il mio, la mia, i miei, le mie

tu: il tuo, la tua, i tuoi, le tue

lui/lei: il suo, la sua, i suoi, le sue

noi: il nostro, la nostra, i nostri, le nostre

voi: il vostro, la vostra, i vostri, le vostre

An exception is the third person plural (they): 

il loro, la loro, i loro, le loro

The form of the possessive particle never changes. 

Another exception is immediate family members. Here the definite article is not used unless in the plural. 

"Mio fratello" / "My brother"

"i miei fratelli" / "My brothers"