From Stresa, you can take a boat to the Borromean Islands. There are different boat companies, but Navigazione Laghi is the biggest and as far as I know the only one going to all three islands. You can just hop on and hop off.
There are different tickets available, we used the Free Circulation Ticket which costs 16,90€ and allows you to use the boat as much as you want for one day. The ride from Stresa to Isola Bella is only ten minutes, from Isola Bella to Isola Pescatori five minutes, from Isola Pescatori to Isola Madre twenty minutes, and back to Stresa thirty-five minutes. In high season, boats leave every thirty minutes.
I found the boats very easy to use, at every stop there are signs indicating the next time a boat will leave and the stops it will go to. Be prepared, though: In high season there are very, very long queues that start to form already about twenty minutes before departure time. Thus, waiting for the boat is not nice and boarding is quite stressful.
Apart from this, it was a nice experience, though! The boats are very spacious with plenty of seating inside and outside. If you choose to sit inside, there are very big windows so you still have good views of the lake.
If you are travelling around by train, Stresa is the nearest train station to the Isole Borromee.
Coming from Milan, as we did, you can either take a fast EC train, or a slower regional train. Because of budget reasons, we opted for the regional train. A one way ticket for one adult is 7,40€ and the journey takes about an hour. You can buy a ticket at the ticket machines in Stazione Centrale, it is very easy. There are different language options, including English, and you just need to follow the instructions of the machine.
Buying a ticket in Stresa for the journey back to Milan, we preferred buying the ticket at the ticket office because the ticket machine looked a bit old. In the evening, the regional trains do not go to Stazione Centrale, but only to Milan Porta Garibaldi (Milan GP), a smaller train station. This was not a problem for us because Milan GP is on the metro, it is only two metro stops from Stazione Centrale and thus it is very easy to connect to the city centre.
Stresa's train station is quite small, but there are free public toilets as well as a pleasant café where we had a coffee and a tea when we arrived in the morning. It is only a short walk from the train station to the town centre - walk down the stairs opposite of the railway station and then just follow the street to the lakefront.
Isola Bella is connected to Stresa, Arona, Pallanza, Locarno, Laveno by the boats of the Navigazione Lago Maggiore company. I should mention that Isola Bella is not close to Rome (what is shown on the map of this page), it is by the Lago Maggiore, about 98km from Milano. When you take a boat to Isola, you see the nice landscape of a lake with green trees and very beautifull vilas! For prices and timetable of boats, you can visit the site http://www.navigazionelaghi.it/ You will find lot of information about lake transport there, including special offers for groups, etc. These are the official boats. You can also try the private offers, although I am not sure how much do they cost. When you go in Stresa, the boat drivers are comming to you and offering their services, you will not need to look for them, they will find you.