Lake Garda is really easy to get to, either by car or by train.
Firstly, it is the Milan-Venice direction that leads you to the lake, from anywhere that you come from and take the Sirmione exit, which, really comes out in San Martino della Battaglia and not on the peninsula, Peschiera del Garda or Desenzano del Garda.
Secondly, you have several options. If you arrive from the west, the most convinient station is in Desenzano del Garda, from the east, the station recommended is Peschiera del Garda.
From here the choice is wide: take a bus, taxi or walk to the centre, which is located less than a kilometre away, and take the boat (see: ferry service).
In any case, for any questions, information or to purchase tickets you can visit the Trenitalia site.
The nearest airports:
Gabriele d'Annunzio, Montichiari, 25km; Orio al Serio, Bergamo, 90km and Valerio Catullo, Verona, 30km. This is connected to the Verona train station by a shuttle bus service available every 20 mins. More distant airports include: Malpensa, Milan, 175 km; Linate Milan 135km and Marco Polo, 155km.