To do this you will either have to wait a long time, or risk many, many weeds (you can get rid of them all with wisp though).
First you have to actually have a decent amount of bells which you have to then place in your bank account. You can wait over time for the bells, or you can change the time setting to the future where you will have suddenly lots of money.
You don't
The money in Animal Crossing is called bells.
Sell fruit, fossils, sea shells, anything.
you save your money
It's Animal Crossing's currency. Bells are used to pay off your debt, buy furniture, clothing, accessories, etc.
15,000 bells
round about 800 bells
Nope.
you save your money
Save up.
15000 bells.
No, but Animal Crossing: Wild World you can get a AR DSi and get infinte bells.