Instructions
Acquiring Rods
Go to Dewford Town, and head for Brawly's Gym. Talk to the fisherman there, and he will ask you a question. Answer "Yes" to this question, and he will hand you the old rod.
Head for Mauville, and walk right once you arrive there. Stop when you come to the water, and surf over to the other side. Speak to the fishermen there, until one of them gives you the good rod.
Go to Mossdeep Town, and walk over to the women standing beside a rock. Turn left and walk until you see a man staring at the water. Walk into the house next to this man, and speak with the fisherman inside to get the super rod. Each of these rods can catch a greater selection of fish than the previous one.
Fishing
Cast your fishing line into the water and wait, just like in real life. A message box will appear with a series of dots in a row, which will look like this: ".............." This is prompting you to prepare to press the "A" button.
Hit the "A" button, as soon as the message box reads: "Oh! A bite!" If you are too late or too early, the message box will read: "Not even a nibble." If this happens, you must attempt the fishing process again.
Press "A" at the right time, and the message will say: "A Pokemon's on the hook!" This means you have captured the fish Pokemon or water Pokemon. Now you can battle it for the right to keep it.