First time - Need help with itinerary

When is the trip? How many nights will you have in Europe? What is your origin airport?

You'll probably do best to fly into Paris and out of Rome or the reverse, depending on time of year. That's a "multi-city" flight that, with luck, won't cost you any more than a round-trip to Rome.

It takes more than 10 hours to travel from Florence to Paris by train, so you'll probably want to fly. You can look for flights on skyscanner.com There may be more options from Pisa than from Florence; I haven't checked.

Start in Paris and end in Rome. Book asap.

Paris is cooler in temperature than Rome, so I would place Rome at the beginning of your trip if it’s occurring up to Summer and head to Rome at the end if you’re traveling in the Fall.

Looking at cheap flights with Vueling between Florence and Paris, and I note the more convenient time for you is a Paris morning departure out of Orly.

So fly in to Paris CDG
Fly Vueling from Paris Orly to Florence
Train to Rome
Fly home from Rome

Here's my standard clarification: To Fly into Paris and home from Rome requires using a multi-city search function, not one-way tickets. Or do it in reverse order. Florence has a fair-sized airport but it is not home to as many intercontinental flights. For European flights, www.skyscanner.com

You did not indicate how many days you would be on your trip.

My third trip to Europe was for three weeks and we started in Rome, with a one overnight stay in Capri, then up to Florence, Venice, Berchtesgaden, Germany and on to Paris. We used the trains to go between the cities, but that was over 30 years ago when flying within Europe was very expensive.

I suggest the following nights for your trip:
Rome 5
Florence 4
Venice 2
Paris 5

I added Venice which you can delete, but the city is very special and should be visited.

You could fly from Venice or Florence (if you skip Venice) to Rome, or you could take an overnight train. That is what we did, taking second class couchettes, which worked out just fine.