You have to get enough points. You do this by buying things in nooks.
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The first shop he has when you start the game is Nook's Cranny. The next shop he has is called Nook 'n Go. After that, he has Nookway. The last and final upgrade of his store is a grand, 2-story building called Nookington's. On the second floor, his 2 nephews, Timmy and Tommy, also work there.
nook (aka pacifer)
Nick Nook Neck
To get a net from Animal Crossing Wild World, you first must have 500 bells (the currency in Animal Crossing), which is the cost of the net. You must go to Nook's store (either Nook's Cranny, Nook 'N' Go, Nookway, or Nookington's), and there should be a shelf dedicated to tools, including but not excluded to the fishing rod, net, and shovel. Then, press the 'A' button on the item, while facing the item. Tom Nook will tell you that it's a net that sells for 500 bells. You then choose the 'buy it' option, and that is how you get a net on Animal Crossing Wild World.
* nook (another word for pacifier) * nipple * night light (for late night feedings)