How do you fish on Pokemon diamond and pearl?

To fish on Diamond and Pearl, you must have a fishing rod. The Old Rod can be found at the little building to the West of Jubilife city. The Good Rod can be found at a fisherman on route 209 near Solaceon Town. The Super Rod can be found at a fisherman who is right above the boats near fight area. To use your fishing rod, go to the key items section of the bag when there is water right in front of you. When an exclamation mark appears above your head, press A and battle a fish.