To set your town to "Keep Town Beautiful" ordinance: go to your town hall, sit in your mayor chair, and choose to set a town ordinance and pay 20,000 bells. The ordinance goes into effect the next day at 6:00 AM.
If you're asking about how to achieve "perfect town" status, here's a some information (from Liquefy's AC:NL guide):
The town rating depends on three general areas:
1) cleanliness
2) abundant nature
3) abundant living
Have between 110 and 200 trees in town.
Have at least 76 flowers in town.
Try building at least 10 Public Works (not counting Dream Suite, Museum Renovation, Fortune-Teller's Shop, Station Reconstruction, or Town Hall Renovation).
Try to prevent villagers from moving out of town. You may have to build more Public Works if a villager moves away. It's possible that the game includes the number of villagers in town as part of "Abundant Living."
Do not allow 10 or more weeds in the entire town.
Do not leave 10 or more items on the ground.
Do not leave garbage on or buried in the ground.
Here are the Public Works that affect Nature Points:
Greatly increase:
...Flower Clock - 87,000 Bells (triggered by "perfect" status)
Largely increase:
...Flower Bed - 26,400 Bells (suggested by Normal female villager)
...Flower Arch - 87,000 Bells (suggested by Normal female villager)
...Wisteria Trellis - 124,000 Bells (suggested by Uchi female villager)
...Rack of Rice - 17,400 Bells (suggested by Cranky male villager)
...Round Topiary - 93,000 Bells (Weeding Day)
...Square Topiary - 93,000 Bells (Weeding Day)
...Tulip Topiary - 93,000 Bells (Weeding Day)
Slightly increase:
...Solar Panel - 126,000 Bells (suggested by Jock male villager)
...Wind Turbine - 156,000 Bells (suggested by any villager)
...Windmill - 372,000 Bells (suggested by Uchi female villager)
Slightly decrease:
...Picnic Blanket - 39,800 Bells (suggested by Uchi female villager)
...Traffic Signal - 82,000 Bells (suggested by Jock male villager)
...Illuminated Heart - 136,000 Bells (suggested by Peppy female villager)
...Illuminated Arch - 148,000 Bells (suggested by Snooty female villager)
...Illuminated Clock - 146,000 Bells (suggested by Peppy female villager)
...Illuminated Tree - 128,000 Bells (suggested by Peppy female villager)
...Tower - 726,000 Bells (suggested by Snooty female villager)
Largely decrease:
...Tire Toy - 49,800 Bells (suggested by Lazy male villager)
Greatly decrease:
...Garbage Can - 53,000 Bells (suggested by Normal female village)
...Video Screen - 284,000 Bells (suggested by Jock male villager)
...Drilling Rig - 298,000 Bells (suggested by Cranky male villager)
Here are the Public Works that affect Living Points:
Greatly increase:
...Campsite - 59,800 Bells
Largely increase:
...Modern Police Station - 264,000 Bells (suggested by any villager)
...Classic Police Station - 264,000 Bells (suggested by any villager)
...Cafe - 298,000 Bells (triggered by Blathers)
...Cobblestone Bridge - 128,000 Bells
...Suspension Bridge - 128,000 Bells
...Brick Bridge - 224,000 Bells (suggested by any villager)
...Modern Bridge - 224,000 Bells (suggested by any villager)
...Wooden Bridge - 298,000 Bells (suggested by Cranky male villager)
...Fairy-Tale Bridge - 298,000 Bells (suggested by Peppy female villager)
...Yellow Bench - 30,000 Bells
...Blue Bench - 42,500 Bells (suggested by Jock male villager)
...Wood Bench - 42,500 Bells (suggested by Uchi female villager)
...Metal Bench - 42,500 Bells (suggested by Peppy female villager)
...Log Bench - 38,000 Bells (suggested by Uchi female villager)
...Modern Bench - 52,800 Bells (suggested by Snooty female villager)
...Fairy-Tale Bench - 52,800 Bells (suggested by Normal female villager)
...Zen Bench - 52,800 Bells (suggested by Cranky male villager)
...Street Lamp - 39,800 Bells
...Round Streetlight - 39,800 Bells (suggested by Peppy female villager)
...Streetlight - 42,800 Bells (suggested by Snooty female villager)
...Modern Streetlight - 64,000 Bells (suggested by Snooty female villager)
...Fairy-Tale Streetlight - 64,000 Bells (suggested by Normal female villager)
...Zen Streetlight - 64,000 Bells (suggested by Cranky male villager)
...Outdoor Chair - 23,400 Bells (suggested by Normal female villager)
...Hot Spring - 98,000 Bells (suggested by Snooty female villager)
...Fire Hydrant - 32,600 Bells
...Stadium Light - 236,000 Bells (suggested by Jock male villager)
...Solar Panel - 126,000 Bells (suggested by Jock male villager)
...Lighthouse - 372,000 Bells (suggested by any villager)
Slightly increase:
...Drinking Fountain - 39,800 Bells (suggested by Normal female villager)
...Instrument Shelter - 42,000 Bells (suggested by Jock male villager)
...Sandbox - 36,000 Bells (suggested by Lazy male villager)
...Water Well - 148,000 Bells
...Fountain - 99,800 Bells
...Tire Toy - 49,800 Bells (suggested by Lazy male villager)
...Jungle Gym - 69,800 Bells (suggested by Lazy male villager)
...Park Clock - 42,000 Bells
...Modern Clock - 78,000 Bells (suggested by Snooty female villager)
...Fairy-Tale Clock - 78,000 Bells (suggested by Normal female villager)
...Zen Clock - 78,000 Bells (suggested by Cranky male villager)
...Balloon Arch - 86,000 Bells (suggested by Lazy male villager)
...Picnic Blanket - 39,800 Bells (suggested by Uchi female villager)
...Hammock - 32,000 Bells (suggested by Lazy male villager)
...Fire Pit - 46,000 Bells (suggested by Jock male villager)
...Camping Cot - 32,000 Bells (suggested by Lazy male villager)
...Torch - 29,800 Bells (suggested by Jock male villager)
...Zen Garden - 148,000 Bells (suggested by Cranky male villager)
...Geyser - 98,000 Bells (suggested by Uchi female villager)
...Statue Fountain - 88,000 Bells (suggested by Snooty female villager)
...Stone Tablet - 39,800 Bells (suggested by any villager)
...Water Pump - 42,000 Bells (suggested by Lazy male villager)
...Bell - 86,000 Bells (suggested by Peppy female villager)
...Zen Bell - 86,000 Bells (suggested by Cranky male villager)
...Scarecrow - 22,400 Bells (suggested by Uchi female villager)
...Bus Stop - 136,000 Bells (suggested by Uchi female villager)
...Traffic Signal - 82,000 Bells (suggested by Jock male villager)
...Custom-Design Sign - 40,000 Bells
...Face-Cutout Standee - 50,000 Bells
...Caution Sign - 43,500 Bells (suggested by any villager)
...Do-Not-Enter Sign - 43,500 Bells
...Yield Sign - 43,500 Bells (suggested by any villager)
...Cube Sculpture - 265,000 Bells (suggested by Smug male villager)
...Archway Sculpture - 265,000 Bells (suggested by Peppy female villager)
...Chair Sculpture - 265,000 Bells (suggested by Smug male villager)
...Illuminated Heart - 136,000 Bells (suggested by Peppy female villager)
...Illuminated Arch - 148,000 Bells (suggested by Snooty female villager)
...Illuminated Clock - 146,000 Bells (suggested by Peppy female villager)
...Illuminated Tree - 128,000 Bells (suggested by Peppy female villager)
...Video Screen - 284,000 Bells (suggested by Jock male villager)
...Parabolic Antenna - 148,000 Bells (suggested by Smug male villager)
...Wind Turbine - 156,000 Bells (suggested by any villager)
...Windmill - 372,000 Bells (suggested by Uchi female villager)
...Tower - 726,000 Bells (suggested by Snooty female villager)
...Stonehenge - 698,000 Bells (suggested by Smug male villager)
...Totem Pole - 538,000 Bells (suggested by Smug male villager)
...Moai Statue - 538,000 Bells (suggested by Smug male villager)
...Pyramid - 698,000 Bells (suggested by Smug male villager)
...Sphinx - 698,000 Bells (suggested by Smug male villager)
...Flower Clock - 87,000 Bells (triggered by "perfect" status)
Greatly decrease:
...Garbage Can - 53,000 Bells (suggested by Normal female village)