1. To indicate a foreign word or phrase:
'I'm fed up with the lot of you. Basta!' Silvio said.
2. To indicate the title of a book or film:
'Have you read A Farewell to Arms?' Catherine asked Edward. 'I hear it's good.'
3. To show the intended stress:
'I thought it was you,' Tiffany shouted at Roland, indicating the car crash.
One to use rarely - it should be clear from the context where the stress should be.
4. To show a different form within the prose, eg a letter.
She took out her notebook and started to write.
Pool - bridge over? Or stepping stones? Rocks - check cleft. Pilgrim's Progress? Date? Reference? Seven steps down. Significant?
She sucked on her pen, trying to think what it reminded her of.
5. To show interior thought:
Everything desirable for men seemed related to size. Bigger car engine, bigger skyscraper, bigger boobs. She looked down at her own. Perhaps I should get a Wonderbra.
'Would you -' She caught her breath. Go on, say it. 'Would you like to come in? For coffee?'
BUT....both the above quotes were taken from the US edition of Nice Girls Do. Italics are liberally scattered throughout the book, none of which I put there, and none of which are present in the UK edition. I checked out a couple of the other foreign editions of NGD and didn't see much presence of italics. Different countries, different styles.
If you're working for a specific publisher they may provide you with a house style sheet to follow. If you're writing on spec, then read some books in the genre and try to get a feel for when/how much they use italics.
PS I was going to write about I for Imagination, but I couldn't think of anything to say.