Keep hitting rocks with an ax or shovel until finding a special one that gives a bag of Bells. Keep hitting it until the bags stop appearing. *** This only happens once per day.
A good way to get started in the game is to purchase a fishing rod or bug net. Set out to catch fish. If it is rare or at least worth 1,000 bells, keep it. If not, throw it back. You can also collect seashells of the same value. If catching bugs, catch them by listening to their sounds or watch for their colors. You can also look for treasure or many other things to collect money.

At 12:30 on any day, walk along the beach shore and look for fruit.
Any fruit you find along the shore is worth 500 Bells.

Dig up fossils in your town. Have Blathers check them, then sell them to Nook. Most are worth up to 2,000 Bells.
Get the Golden Shovel and use it to bury money. It may grow into a tree that will grow Bells, depending on how much money was buried.
Go to the bank and access your account. Save all the money you have into the account. Save the game and turn off the DS. Change the date to 10 years later and your money will go up, depending on how much is in there. It usually goes up 99,999 Bells.

Take your Golden Shovel and dig in random spots. Eventually you will dig up 100 Bells.

Find a star on the ground. Dig it up with your shovel and sell it to Tom Nook. If it is a fossil, chat to the museum owl. Make sure you do not donate it, even when he asks if he can keep it. Sell it to Tom Nook for a few thousand Bells.

Change the system date to January 1st to get mail from your Mom with 10,000 Bells.
Insert any amount of money into your savings account, as long as it is about 50,000 Bells. Save the game and turn off the Nintendo DS.
Go to the DS settings and change the date to the year 2099. When you resume the game, you will have a letter stating that you have received 99,999 bells. This has automatically been added into your account. *** You will receive many weeds. *** It is better if you change the DS date to the year 2000, play the game, save, then
change it to the year 2099. You will get best advantages by doing this. If this did not work, it may be because of several reasons.
For example, you may not have earned interest in your savings.

To get more money for fruit, go to your friend's town. Instead of
only 100 Bells you will get 500 Bells.

When Joan the turnip peddler comes to your town (every Sunday morning), ask her if she has Red Turnips. If she sells them to you, plant them immediately. Water them every day for a week, and dig them up on Saturday. They will sell at Nook's shop for 16,000 Bells.

Buy a Fishing Rod and fish at the ocean around night time when it snows or rains. When you see a giant fish shadow, it will be a living fossil fish, worth 15,000 Bells.

Every day, go to the beach and look for shells. Do not bother picking up the Sand Dollars, Dall Tops, Venus Combs, or anything else worth less then 500 Bells. Just pick up the shells called "White Scallop" and "Pearl Oyster". They are the rarest kind. Also when you check, you might find a Coconut at the beach shore, or a Otter (sailor), and he might give you some random furniture.

Go to an online friend's town. Pick their fruit and plant it in your town. You can make more money from the fruit when it grows.

Go to a friend's town and pick their fruit and grow it in your town. Dig out a weed and plant it there, or chop down a tree and it will grow 50 to 100%. If you plant it when it is crowded, it will only grow to be a short (tiny) tree. You can sell the fruit to Tom Nook for 500 Bells each.

Whenever you get a fruit you do not have in your town, plant it.
After you have planted it, water it. This even works on coconuts that you find on the beach, but they need to be watered more.

Plant a fruit from another town in your town (not your native fruit). Keep planting them until you have an orchard (fifty or more trees). You should have your pockets empty when you go to collect
them (to maximize efficiency). After your pockets are full, sell
then to Tom Nook for 500 Bells each. You can get 7,500 Bells for
each pocketful of fruit.

Shake trees until a hive of bees appear. Now, take out your net and
catch them. Each bee is worth 4,500 Bells.

Take out your shovel and hit rocks with it until it spits out Bells.
Keep hitting it. It can go up to 20,000 Bells. If you want to get
all the bells, dig 3 holes away from the rock, and hit it. This
will push you back but you will still hit the rock because the holes
are there.

Eventually you will find a coconut. Your mom may also mail you and
include fruit with the letter. Bury them. After five to 7 days,
they will become trees. Shake them, and you will get more of that
fruit. Sell them to Tom Nook, for 500 Bells.

To get coconut trees, find a coconut on the beach then plant it
within 10 shovel spaces from the ocean. Keep planting the fruit you
get from the trees. Soon will have a friendly tropical beach. You
can also sell the coconuts for 500 Bells.
Get a fruit that is not native to your town. Plant it and go ahead
in time Bury those fruits you find and go ahead in time again.
Repeat this, then eventually sell all the fruit.