I like the suggestion to check out the Man in Seat 61 to read just a little bit more about the options. The fastest seems to go via Villach and the other via Innsbruck (also sometimes referred to as the Venice/Munich route). I like the scenery on the Innsbruck route and would probably choose that although it is a bit longer than via Villach. However, the direct Innsbruck connections may not line up with times you have in mind.

I think this link in the Man in Seat 61 takes you directly to the Venice/Salzburg options. Once you decide, you can follow the recommendations of others on how to book. I believe there are direct links to ÖBB from the Seat 61 website. See which times suit you best and go from there.