Here's some money-making tips.
1. Every day, one rock will be picked randomly to be the money rock. Smash it with the shovel repeatedly to get about 7 Bags of bells, doubling in amount every time (100, 200, 400...). Unfortunately, while pounding away you get pushed back. If you stop smashing the rock, it won't give you any more money. The solution here is to dig holes to limit you movement.
Here's the best way. This means that the money can't be blocked by any holes.

2. Shells, Fish and Bugs. Sell them to Tom Nook.
3. Fossils. If you get any more that have already been donated then sell them - you get lots of money.
4. After new years, your mum will give you money in a letter (I got 10000 Bells!).
You'lle be rich in no time.