Train from Munich to venice

Where did you buy them from? And what delivery option did you select?

Who did you buy the tickets from?
What option did you select when you bought them?

If you bought them direct from DB, you probably have "print-at-home" tickets (it will have asked you what option you want when you bought, before you paid).
In which case, your confirmation e-mail has an attached PDF file. You need to print this out, this is your ticket.

I think the conclusion is: read what you are buying and ask these questions before you enter your credit card details.

If your tickets are "print at station," then probably any ticket machine near the track area in the Munich main station can print them out based on some form of ID or code. But your confirmation email should have instructions.