I echo what the others here are saying. Making my own tarantula islands got me off to a great start. You can also dig up the manilla clams on the beach and make fist bait to have an easier time fishing. If it's raining, I highly recommend this, as you have a better chance to catch Coelacanth which is worth a nice return, particularly from selling to C.J. when he's in town. Plant lots of fruit not native to your island and sell them - each non-native fruit nets you 500 bells. Another good thing to do is if you see a villager crafting a DIY recipe, try opening your gates with a Dodo code and posting here. Most people are very generous and will tip, and you can request tips in IGB to help. It's a nice little side gig if you have the time and patience for it.
Once you have a decent amount, try buying turnips on Sundays before noon. With the help of this forum you have a great chance of finding an island buying at 500+ bells per turnip (or your shop might even be buying at this rate!). This is how I made most of my bells, as an investment of just 200,000 when the original price is less than 100 bells can give you over 1,000,000 if you sell them above 500.
In fact we're not allowed to use or sell cloned items on this forum. Some suggestions we have would be to buy turnips tomorrow and sell them at a higher price through nooks! Catching bugs, fish and selling fruit is another way to get some good bells! For example, some people might be willing to trade some fruit and planting it in the ground will grow new fruit! Fruits that aren't your native will sell for 500 instead of 100 so you can get a nice sum if you plant enough!
