I just did an itinerary for first-timer friends. Being in your 20's the trip should be a breeze with a Euro railpass (travel first class and save money by not renting a car). I'm from LA too, but I would definitely skip the driving on your first trip. Italy is our favorite country in the world but it is a "different world" compared to the rest of Europe!! Rick saves you tons of time and tons of dollars and tons and tons of hassles. Get Rick's "Pocket" books on Venice , Florence and Rome - these act as your tour guide inside the major sights meaning you can follow Rick's "masterpiece" map through the Vatican Museum in two hours instead of two weeks (we're talking 4 miles of art here!). He also has one on CT as well that we have used. His stuff is great, I can not emphasize this enough. Ninety percent (no exaggeration) of the tourists you'll see will stand in LONG lines and once inside will not have a clue as to the significance of what they're looking at, RS brings all this to life in synopses in the book you carry. Note: use your iPhone for pix and forget about trying to read it or an iPad with downloaded books. They get cumbersome, are hard to read because of glare and are thief magnets who run by you swiping it from your hands.
Can you tell we've experienced all of these things! I'm not trying to tell you what to do, of course, but I know how different Italy is than the rest of Europe - it's fascinating but can be incredibly frustrating on your first trip when the hassles start building up when you always seem to be lost, behind schedule, and the money is burning up fast. Some more important tips: carry all your luggage on/use the kind that has backpack straps (one suitcase, one small bag only - no one cares what your wearing); wear a money belt; follow RS's guidelines for these items. More tips: use Airbnb for lodging, Rick or the locals for food and keep a small daily journal to jog your memory later - stuff starts running together fast! Buy the Roma card and Firenze card and make a Resv. at the Borghese (don't miss this) in Rome before leaving the US.
OK, suggested itinerary: Venice 2 nites, Florence 4, Assisi 1, Rome 4, Siena 1, CT 2
You can add a nite in Venice if you arrive late; Flor to Assisi is on everyone's list of "scenic train" trips;
It's cheap to fly in and out of Geneva or Zurich (pricey lodging tho). If you "redeye" in you can be on a train in less than 30 minutes traveling some of the most scenic trips on Earth (Switzerland and Austrian Alps) on your way to Venice and leaving from CT. Use Ricksteves.com, plan as early as possible and print your tix at home for major sights. Misc: Rome-don't lodge to the west or south of train station; in Flor it's a no on the NW side. Taking a siesta, especially on long hot days, charges the batteries fast for strolling at nite and eating at 9pm. The nights in Venice, Florence and Rome (actually almost everywhere in Italy) are very unique, special and dare I say, magical. Yeah, I do say so. Ciao Bella!