Yvonne, as you can probably see from the map the rail line from Venice only skirts along the south shore of the lake with 2 stations at Peschiera and Desenzano.
This may sound o.k. but the real appeal of Lake Garda lies further north along either shore. If you do not wish to transfer to the bus, say at Verona, you could base yourselves at Desenzano and make trips up the lake each day.
I would favour Des' because it is the boat depot for the whole lake so you would get a good choice of options. If you were intending to stay for a week or so then it would certainly be worth while relocating to one of the nicer resorts such as Garda or Malcesine.
By the way I always wondered, can you buy good maps of Italy in Oz.?
Good luck,
Bob.