Hannah,
As this is your first time in Europe, I'd suggest reading Europe Through The Back Door prior to your trip. That provides a lot of information on how to travel well in Europe. You may also want to have a look at the RS Italy guidebook.
Some additional information would be helpful. For example where are you flying from and when is this trip taking place? I note that you have quite a number of other posts, but I haven't read all of them. It would help to see an overview of your entire Itinerary.
If the Dolomites are going to be your first stop, I'd suggest spending one night either in Milan or Verona, to get over jet lag and get your bearings. I wouldn't want to make the trip from MXP to Castelrotto after a long international flight, but from Verona it's an easy trip to Bolzano. You'll need to do some research on using the trains, as in some cases you'll be making changes, and there are a few potentially expensive caveats to be aware of.
You might consider staying in Castelrotto for a few days, as that's a good "home base" for exploring that area. As mentioned in the previous reply, there's a bus from there to the cable car station at Seiser Alm / Alpi di Siusi. You might find this website helpful - https://www.seiseralm.it/en/summer-dolomites.html
