How do you get to the Milan Canals from the Duomo?
You might try googling "Navigli" which is what the canal area is called. You can get there by a tram, but unfortunately I can't tell you which one (my visit to that area was 15 years ago).
Take the 3 tram (south bound) from west side of the piazza.
edit- And the 2 tram will take on to the western part of the Navigli.