First, you must go to shell beach south of central park. There you will find an old man with a rod of his own. Talk to him and he will say he will make you a rod if you bring him a lure, rod and thread. You find the lure at Divermon lake. One of the Divermon has the red snapper lure stuck in his throat. Help him out and he will give it to you. The rod and thread are pretty simple to find. Go to Smith's shop in the beginning city and get a bamboo spear,which will be the rod. The thread can be found in Protocol Ruins. When you battle the Dokugomon there they sometimes leave the item spider web. That becomes the thread. Take these items to the guy and he will make the rod. With the rod you can cast in almost any water. When you do a bar appears at the bottom of the screen that will have sections of yellow, blue, and/or green. Stop the cursor that appears on the bar at blue and you don't catch anything. Stop it on green and you will battle some sort of water Digimon. Land on yellow and you shall battle a blue Cardmon that sort of looks like a fish. Beat him to get a booster pack of cards for card battles, but watch out for his curse attack.