I wrote this whole thing about the usage of the definite articles, but now I wonder if you meant the pronouns... well, here's the first part anyway.
Masculine words
il (plural i) - beginning with most consonants (see exceptions under “lo”)
il libro, il telefono, i ragazzi, i vini
lo (plural gli) - consonants sp, sc, str, sm, z and gn.
lo spagnolo, lo scappolo, lo zoo, gli uomini, gli studenti
l' - beginning with any vowel
l’aeroporto, l'amico
Feminine words
la (plural le) - feminine word beginning with consonants
la donna, la piazza, le cartoline, le parole
l' (plural le) beginning with any vowel
l'amica, l’americana, le amiche