Every Italian noun has a gender. The ending is usually a good clue:

Ending	Gender	Examples
-o	mostly masculine	libro (book), gatto (cat), amico (friend)
-a	mostly feminine	casa (house), porta (door), amica (friend)
-e	either — learn it with the noun	il fiore (flower, m), la chiave (key, f)
-zione	feminine	la stazione (station), la colazione (breakfast)

A few famous exceptions: la mano (hand) is feminine; il problema and il programma are masculine. Learn each noun with its article and you'll never have to guess.