How do owls in Potterverse know how to deliver letters? Is there some sort of training they go through before being out in the field? Can any owl be used, or are there only specific breeds that are more suited for the task?
I haven't found any explicit mention of owl training in the books yet, but in the sixth movie the Quibbler classifieds lists "Owl Training" (here, on the upper left), and I'm not sure that witches and wizards would need to train owls for anything other than delivering mail.
As for the owl breed, there's no indication that some owl breeds can't be used to deliver mail, but certainly some are better at it than others. Hedwig, for example, is an exceptionally intelligent and adept owl (as noted by a number of people in the books) given that she somehow knew that Harry had gone to the Leaky Cauldron in the third book, and that she could always find Sirius in the fourth book. This seems to be a sort of sixth sense of direction that owls have, just like Thestrals. There's speculation that owls must have a special affinity to magic for this sense of direction, and that not all owls do.
Also, obviously a stronger owl is generally more suited to long distance mail while smaller owls can't travel as far. We see this in the stark contrast between Pigwidgeon, who considers it a great feat to deliver a small letter from the Burrow to Hogwarts, and Hedwig, who can regularly make several day long trips without the slightest difficulty (and who clicks her beak with contempt at the way Pigwidgeon flutters around with joy after a delivery). Also in the third book, we hear of the owl postal service in Hogsmeade which colour-codes owls based on how far they can travel and how long it will take for them to arrive at their destinations.
So I'd say that owls most probably need to be trained, but some can have particularly good navigational intuition that just makes them better at delivering mail. Owl size also matters, but the breed shouldn't, I think.