Lake Garda is massive — 52km long, three regions, two vibes. Southern shore (Desenzano, Sirmione) is flat and easy by train. Northern tip (Riva, Torbole) is Alpine and needs a car.
Milano Centrale → Desenzano (55 min fast, 1h15 regional) or Peschiera (1h05). Both right on the lake. Bus or boat to Sirmione from Desenzano (20 min).
For western shore (Salò, Gardone, Gargnano) or northern shore (Riva, Torbole), a car is essential.
Train for south shore. Car for western/northern shores. Lake Garda is too big for one transport mode.