How do you get bells?

There are various methods you can use to attain Bells (currency).
Catching and selling fish and bugs, shaking fruit trees and gathering the fruit to sell, collecting shells on the beach, collecting and selling hybrid flowers, locating your town's daily gem/cash rock, planting/growing perfect or foreign fruits or bamboo, and/or selling furniture given to you by the neighbours in your town. It's advisable that you sell things at your town's store, Re-Tail, as you'll get more money from there and much less from Timmy & Tommy Nook in their store on Main Street.

Shaking random trees around your town has a chance of yielding bells, a beehive or a piece of furniture. If you can catch a bee, they sell for 2,500 Bells apiece plus an extra 500 Bells per beehive. There are only 5 beehives generated in your town per day. You cannot catch bees in other players' towns or during online play.

If you have access to the tropical island, each night from 7PM, rare bugs and fish will spawn. This is the best place to make money due to the permanent summer state, but it does require some patience. You can also do this with a friend.