Italian diction is fairly simple for native speakers of English, and is a lovely to sing, in part because of its purity of vowels. This is one of the reasons why many teachers have their students sing in Italian.
The Diction Domain is a good place to get started – Diction Domain Italian. You might also want to download this handout.
A FEW NOTES ON ITALIAN DICTION
DIPHTHONGS
When two vowels appear consecutively within a word in Italian, each of them is pronounced, producing a dipthong. See Diction by John Moriarity and Diction for Singers by Joan Wall for more information.
TRIPHTHONGS
When three vowels appear consecutively within a word in Italian, each of them is pronounced, producing a triphthong. See Diction by John Moriarity and Diction for Singers by Joan Wall for more information.
DOUBLE CONSONANTS
In Italian, there is a big difference in sound between single consonants and double consonants. Double consonants generally fall in the middle of a word, but can also occur when a word ends with a consonant and is followed by a word beginning with the same consonant. The difference is basically this: in English, when speaking a word with a double consonant, the speaker will end the first syllable with the vowel, and begin the second syllable with the consonant. An Italian would end the first syllable with the consonant, and begin the next syllable with the consonant. So, for example, an English speaker would pronounce the word pappa /pa-pa/, an Italian would pronounce it /pap:pa/. Attention to this detail is crucial to making yourself understood.