Hi Emily -
Here is a start. Arno and Annika are the experts on rail passes, so i won't comment on those.... except to say that I think the Tell pass is only valid from Zug onward in your journey from Zürich to Luzern.
It is extremely easy to buy tickets in Switzerland at all stations. Whether manned or unmanned, they have easy to use ticket machines, with English as an option. Also, reservations are not necessary INSIDE Switzerland, except for a few special scenic trains. The ticket counter at Zürich airport often has long lines, but they move quickly. I always use the ticket machines, myself.
Some rail passes eliminate all ticketing; a half-fare rail pass requires you to get tickets for each journey; the ticket machines have a clearly marked option for "1/2 fare."
There are good frequent rail connections from Zürich or from Luzern to Milano Centrale, where you will change trains for Florence (Firenze). Allow plenty of time there. Don't count on Swiss-style precisely timed easy connections there. You may not need extra time, but I and other travelers have often needed it.
Those two major rail routes in Switzerland for that journey (from Luzern and from Zürich) cross each other (meeting on two sides of the same platform) at Arth-Goldau. You might have to change at Arth-Goldau, but it is very easy.
The timetable can help you, for journeys to Italy, too.
If you enter Zurich Airport as your start and Luzern ( German spelling) as your destination in the timetable, you will find two trains per hour all day long for the approximately one hour journey to Luzern. They leave at xx18 (with change at Zürich Main station), or at xx47 direct with no change, but a stop, at Zürich main station.
As the xx18 trains may come in to different tracks at Zürich main station, some of which are a few levels down in the station, I'd be wary of a tight connection, especially if you have luggage.
Luggage carts are available; you need a 2 CHF coin to unlock them from their queue; you get the coin back when you "plug" them in at a queue near the place that you are finished with them There are elevators ("lifts" if you are English) and, the luggage carts cleverly go up and down escalators. You can see arrival and departure tracks on the timetable ( click on the "+" to the left of a journey).
Main platform is tracks 3 - 18. Transfers between those are easy. I often advise travelers to Luzern who are not in a rush to stop in the food court above the rail station at the airport and have a coffee, then catch the xx47. Or, since there are many trains per hour from the airport to Zurich main station, take any of those and wait for the next train to Luzern at Zürich main station; maximum wait about 30 minutes.
In Italy, you'll need a ticket from Trenitalia, but you can buy it easily in Switzerland.
English is spoken widely in major tourist centers. In fact, with some foreign (non-Swiss) workers in the hospitality industry, English may actually be preferred to one of the 4 Swiss national languages.
In Luzern, the tourist info office by track #1 is very very helpful.