Click on destinations guide at top of page, enter Italy and you'll find plenty of info. Look at the article on www.wikitravel.org also. Or download guides for some of the cities from www.arrivalguides.com.
Best time depends on what you want to do so maybe you can clarify that. If you want warm weather, July/August are the best but many parts will be very hot, very crowded and very expensive. I would prefer travel in April-June or September-October. If you want to go skiing February is probably best.
Where to go also depends on your interests but highlights for me are Rome, Florence, Venice, Cinque Terre, Amalfi Coast, Sicily, the Alps and especially the Dolomites - one of the most beautiful places I've ever been in. Bergamo is also one of my favourite cities in Italy. Padua well worth a visit. The lakes - Maggiore, Como and Garda are fantastic - have not been to Garda myself yet.
There's just so much to see and do in Italy.