What is the best way to get money?

Fish, fish and Fossils

Sell Fruit.

Well, it depends whether you want to use time travel or not.

Time Travel: As you may know, every month if you have bells in your bank you will earn some interest from it. If you deposit a large amount and time travel many years into the future, you should find that you have earned quite a bit of bells. You can also use time travel to make it constantly Summer so you can catch rare beetles on palm trees.

No Time Travel: If you are someone who can persevere with being repetitive and patient, there are certain things you can do daily to earn bells. If you have a shovel, find the daily money rock, which can spit out a maximum of 8,500 bells (or a chance of 16,500 if you have lots of yellow items on the left side of your house). With this, check every rock in your town. One will spit out bells. If you have some holes behind you, you will not be pushed back, and can hit the rock for maximum bells. Also collecting Fossils daily and taking them to Blathers, then selling them will net you a good amount of bells (also if the previous day it has rained, there should be some Gyroids which sell for roughly 800 bells each). Every 3 days new fruit grows on trees, so you can also sell fruit. You could also try shaking bare trees to find bells and furniture you can sell, and sometimes check the recycle bin in The Town Hall. Throughout the day, you can sell shells on the beach, catch fish and Bugs, and receive items occasionally from your villagers which you can sell.

However in my opinion the best way to earn money is in Summer, catching rare beetles on palm trees. To do this you will first have to have palm trees. To get them, plant a coconut in a good spot on your beach; it should grow into a tree. Then late at night (presumably after 9pm) rare beetles which sell for 6,000-12,000 bells may appear on these trees. Be careful catching them though.

Hope this has helped!