Over whelmed

You do not have near enough time to add Croatia and are giving Italy a very limited look.
Naples is in Southern Italy so save it for a trip to that region of Italy when you can include Sorrento and the Amalfi Coast.
So you plan on three days each for Rome and Florence? Rome needs at least one more day especially if you will land there jet lagged. .
Where are you planning to stay in Tuscany? It is a large Province. You mention using “ the transportation system” but you need a car to explore this largely rural area and can pick one up as you leave Florence. You will find find many wineries and cheese makers there.
A week will give you a chance to explore some of Tuscany and include those activities you are interested in doing. It will be a great introduction to Italy not a quick look over of too many places.

You describe yourself as overwhelmed, and that's easy to understand because Italy offers to many great places to see. Just realize you can't see it all and make peace with that. IMO there are no "must dos" in Italy. You really can't go wrong (except, in my limited experience, Turin city wasn't all that attractive, and almost ditto for Milan -- except the cathedral and the Last Supper -- though many will disagree).

I concur with Suki on most points. Realize that 14 days on the ground means really 12 days to see Italy as you'll be jetlagged on Day 1 and preparing for departure on Day 14. But, I say if Naples and the nearby region (Pompeii, Herculaneum? Amalfi coast?) is important to you, go ahead and fly into Naples, but allow adequate time (like 5 days) to experience that southern region, enjoy the food, etc. In that case I might say to skip Tuscany, because really the south plus Rome will give you plenty to see and do for 12 days. Or, you could go up to Florence and Tuscany and skip Rome -- it is always fairly easy to get there so you could see Rome on a future trip. If you choose Florence/Tuscany and are locked into a homeward flight out of Rome, just arrive in Rome the day before so you get a little taste of Rome and you are near the airport, not stressing about getting there on your day of departure.

Welcome to the forum!

Most would say that for a first trip to Italy-- Naples is not the best place to go--at least not at beginning of trip. Just not an easy entry to Italy.
The typical first trip to Italy is Venice-Florence-Rome and there is a reason for that.
Venice is a great place to get over jet lag and get acclimated to Italy. Train travel between the 3 locations is super easy.
There are direct flights to/from Venice and Rome, probably not to Naples.

It is easier to arrive in Venice than it is to depart from Venice.

You want to count your trip in NIGHTS not days. How many NIGHTS will you have on the ground in Italy? I agree with Suki that you really don't have time for Croatia. Sure it is a great place but so are dozens of other countries/cities in Europe. Stick with Italy.

When planning your itinerary keep in mind that a 2 night stay really means just 1.5 days in that location. You will spend a half day packing up, checking out, getting to train, taking train, getting to new lodgings, checking in and getting oriented. Try to avoid 2 night stays and certainly avoid 1 night stays. (although sometimes a 1 night stay is necessary at begining or end of trip.)

You can take a look at Rick's suggested itinerary here:
https://www.ricksteves.com/europe/italy/itinerary
but keep in mind that most of us here feel that this itinerary is far too fast paced.

Recommended bare minimum stays
Venice- 3 nights (especially if you fly in here- 1st day is jet lag)
Florence - 2 nights add a day for each day trip planned.
Tuscany 3-4 nights needs a car- are you willing to drive?
Rome 3 nights

Most folks don't go to Italy to relax on a beach- in general the beaches are not really as you might imagine, they are mostly rocky (not sandy) and difficult to access.
A stay at Lake Como might be a good substitute.

Figure out where your best flight options are. Often this is RT Rome, which can work. But if at all possible do an open jaw trip- fly into one city out of another- called "Multi-city" or similar on airline website. Ex: Fly into Milan, out of Rome or fly into Venice, out of Rome.

A good place to search for flight options is Google Flights- use it for reference but always book your flight directly with the airline.

If you haven't already- do get RS Italy guidebook, it has all the info you will need.

Hope that helps- planning should be fun not overwhelming.

I’ve spent a collective year in Italy (6 month study abroad and multiple vacations), and I’ve never even been south of Rome. That’s how much there is to do in Italy. It’s completely understandable that you’re feeling overwhelmed. I know that it’s so tough to cut out things you want to see, but realize that less really is more, in terms of the number of cities. Just remember, you will be back!

One thing that helps me when planning is to pull up a Google map of the country and pinpoint on the map where we want to go. Then I can eliminate any outliers and focus on locations that make sense, logistically. For a first trip to Italy with only 12 nights, I’d echo a Venice - Florence - Rome itinerary.

14 days and you don’t want to feel rushed? 1st of all, delete Croatia. Even if the 14 are days on the ground, vs including travel, you don’t have enough time to add another country. Do you realize that you lose a day when traveling from N. America? That is, if you leave on Monday, you arrive on Tuesday. You are also likely to be jet-lagged, so the next day should have low-key plans. The last day is also likely to be fairly useless for everything but packing, checking out, and getting to the airport.
I would have 2 bases; Rome and Florence. If the airline gods co-operate, you could fly into one and out if the other. Otherwise, into and out of Rome. You can do day trips from both.

Given your newbie status, I would recommend that you take some guided tours. There are many postings here about tours of all sorts that people have loved, including site tours, food tours, wine tours . . . the variety is enormous. To hardly scratch the surface, Ostia Antica and Orvieto are great day trips from Rome; and Siena, the Chianti area and Pisa and Lucca, from Florence.

Since you didn't mention Venice, I would skip Venice. Walks of Italy and Tours by Roberto tours may be of interest to you, especially if you don't want to rent a car. If you do rent a car to see Tuscany, be sure to read posts on this forum about driving in Italy-it appears there can be hefty fines if you're not careful where you park. Also do read the info about pickpockets-but on 4 trips, that has never been a problem for us. 3 days in Rome will really feel rushed-the city is very spread out and the subway system there is not as useful as those in London, Paris, and Athens. As someone else mentioned, flying in or out of Florence is an option-when we did it, we connected in Paris and it was quick and easy to get to the airport from the city center by a not-especially-expensive taxi.
I don't know if you've flown to Europe before, but Italy is a significantly longer flight than London, so do expect to be jet-lagged.
You are going to have a great time-many of us love Italy the most!

We just got home from 5 weeks in Italy Sept/Oct 2021. One practical question, we flew American Airlines from DÍA, connecting in Dallas/Ft. Worth, then direct from Dallas to Rome. This was because our originally intended British Airways Denver to London to Bologna got cancelled by the airline. Have you identified how you’d get to Naples from Denver? Flights should be flying more and more in the future, with fewer Covid cancellations - that is, should.

For reference now or in the future, a trip we took years ago, we flew into Venice, then rented a car at the airport and drove east, to visit parts of Slovenia, then down into Croatia. We then returned the car back in Venice for some Italy time.

Other ways of reaching Croatia, which you may have already determined, could be a ferry from the Italian east coast, or flying from Italy. But you’d need to figure from what port or airport to what destination. Then, would you be going back to a italy before heading home, or coming home from Split, or Dubrovnik, or elsewhere in Croatia? “Close” to Croatia might not always mean easy.

Croatia is more doable if going to Venice so I would go there on a future trip. When you’re in Naples do you plan on visiting the Amalfi Coast, Pompeii?

We've had good luck with BA from Denver. From our hard-earned experiences, I try to plan four night in any city that we go to. You arrive and spend the night. Then, you have two full days to explore. After night four, you leave to the next location. After a few hours on a train, rinse and repeat. For fourteen days, that's a max of three locations. There are lovely beaches in Italy. I'd save Croatia for another trip.