Croatia, Venice, Rome

I find it more challenging to plan a trip linking two countries, but it can be done. I always start by writing down each day, something like this:
Arrive Dubrovnik
2-3 nights
catamaran to Hvar
1-2 nights
catamaran to Split
1-2 nights
bus to Plitvice
1 night
bus to Zagreb
1 night
Then, fly to whichever Italian city has direct fights, travel between Italian cities by train. You would want no fewer than three nights per place in Italy.
This is a fast paced itinerary, and I would add more time to every place that you can--these are just minimums.
Use Google to search flights and transport between places.

Obviously, you can do different orders and part of that will depend on what time of year--play around with all of the combinations to see what works for you. Good luck!

It is. In my experience it is extremely easy to fill time in Europe, where it's rare to travel 10 miles without passing a place of interest. Therefore, I'd shoot for as long a trip as you can manage. As you dig into guidebooks, you will surely find more and more places you'd like to go. The shorter your trip, the more painful the omissions will be.

The Italian cities--especially Rome--will be more time-consuming individually because of the sheer number of sights. They are among the most popular destinations in Europe for a reason. Transportation from place to place will be somewhat slower in Croatia because you'll be on buses and ferries vs. the express trains in Italy. The link between Zagreb and Venice is not fast; by train it will be 9 to 10 hours. Buses are scheduled to be about 3 hours faster, but I don't know how reliable they are. They will be nice, modern buses.

I'd recommend getting some guidebooks (likely available from your library). Rick's books are very good on how-to-travel topics as well as providing in-depth information on what he considers to be the top sights. There is some summary sightseeing information on this very website. Go here, click on a city, then on At a Glance.

https://www.ricksteves.com/europe/italy -- for all three Italian cities

https://www.ricksteves.com/europe/croatia -- detailed info available only for Dubrovnik

You can watch Rick's TV shows here: https://www.ricksteves.com/watch-read-listen/video/tv-show. The Croatia show is listed under "Central and Eastern Europe". Many of the videos are quite old, so details may have changed.

Two weeks is roughly the target itinerary for a good Croatia itinerary. You’ll want to time to explore and enjoy each stop. Croatia is a little tricky in that all these destinations are a few hours travel from each other, with limited public transportation connections each day. This makes quick overnight stops a wasteful, draining enterprise. What time of year you plan on going has a huge impact on your itinerary here as well. The coast is a summertime, outdoors destination. That’s when you can really enjoy the incredible beauty, and when these towns really switch on. It’s also much more expensive, but worth it imo. As opposed to places like Rome say, if you’re here in shoulder season and it rains there’s not really any major museums to spend your time in.

I’d encourage you to get ahold of Rick Steves Europe through the Backdoor book. Grab the books for Italy and Croatia. Read up. Watch some shows. This will give you a great background on what to expect, and how to budget and plan your itinerary. Building an itinerary might be the most important step on a trip like this. His style of travel really is a bit of a philosophy, and it’s served me well. And if I can offer some encouragement: Almost 25 years ago, a trip to Europe felt almost unattainable for me. I thought I’d never find the time or the money. With Rick Steve’s tools, I found out pretty quickly that Europe didn’t have to be a once in a lifetime experience. You can take it in chunks, you can (mostly still) do it on a budget. I’ve somehow managed to travel to Europe almost every year since.