To get a golden watering can, you need to maintain a perfect town in Animal Crossing: Wild World for 15 days. Having a perfect town means keeping it clean and having a good number of trees and flowers distributed evenly throughout the town. When you have a perfect town, you will randomly discover Jacob's Ladder flowers in your town. Those types of flowers will only appear if your town is perfect.
Your town is divided up into 16 acres. Each acre is 16x16. What this means is that if you dig 16 holes in a line from the top of the town and 16 holes from the left of the town, you have measured one acre. You can also measure out acres with designs on the ground, or flowers, or whatever. The maximum number of holes you can dig (or flowers you can plant, etc) in an acre is 256.
To have a perfect town, at least 8 of the 16 acres must be considered perfect. You don't have to have any trees an acre for it to be a perfect acre, although you will have to have a bunch of flowers in it to make up for the lack of trees. If you do not want to have to maintain a bunch of flowers all the time, put 12, 13, 14, or 15 trees into an acre, and you will only need to have three flowers in that acre for it to be considered perfect. For each number of trees fewer than 12 or more than 15, add two more flowers to make it perfect. So having 11 trees means you need 5 flowers, and having 10 trees means you need 7 flowers, and so on.
You can have at most 8 non-perfect acres in your town. These acres will still need to have flowers or trees to be considered acceptable if you want a perfect town. An acceptable acre that does not have any trees must have at least 21 flowers. If you have 10, 11, 12, 13, 14, 15, 16, or 17 trees in the acre, you do not need any flowers in it to make it acceptable. Therefore, it is a good idea to make sure all acres in your town have at least 12 trees in them, because then you will only need to put 3 flowers into a given acre for it to be considered a perfect acre. Nine trees in an acre, or 18 trees in an acre, will need 3 flowers to make it acceptable. For each number of trees fewer than nine or greater than 18, add two more flowers.
More than two weeds in an acre will make it unacceptable for a perfect town. More than two items in an acre, excluding fruit, shells, or acorns, will render it unacceptable.
Use patterns on the ground to help you determine the boundaries of your acres. Then consider planting a tree on the boundaries of each acre to give you a permanent (sort of) indication of where the boundary is. After that, plant 12 trees in each acre, then put at least three flowers into at least eight of the acres. It is best to put more than three flowers into those acres, in case you forget to water your flowers and end up losing some to wilting. Be sure to water your flowers every day until you get the golden watering can.
Talk to Pelly or Phyllis in the left-hand window of the town hall and ask about The Environment. She will tell you if the town is perfect or not. Then, on the day after you have had a perfect town for 15 days, she will give you a golden watering can when you talk to her.