@Cerebral Labyrinth dropped some good points, I'll also add.
Fishing/Bug catching - After you get into the groove of being able to catch bugs and fish, you should already note what sells for high to medium to just plain nothing. You should ALWAYS stock the high selling fish/bugs for when either CJ (Beaver cat that fishes) or Flick (Punk looking Dinosaur that catches bugs) show up because they buy 'em for more than the Nook twins do.
You aren't pressed to SPEND money in this game. Always remember that, so short of buying stuff in the shops that rotate, you should be looking to max out your earning potential by selling stuff at the highest price you can get for it. I got rooms in my house dedicated to stacking up boxes of items, waiting to offload when either comes. They help you earn a great deal outside of the Turnip market.
Plant flowers - Yeah, I know it's a chore with all the crossbreeding and weird shyt they never explain, but planting flowers up in the mountains and other area increases the chances of rare butterflies and other shyt floating about. Plant flowers of the same variety one square apart (A dug hole is a square, if you're wondering what I'm talking about) and water them in hopes of crossbreeding, because if it goes well, a new flower will pop up in that square and increase the chances of rarer creatures landing on/around it. I haven't gotten into decorating with flowers yet, but planting them all about the place has yielded some very favorable results.
Create a tree stump in random areas of trees 'cause other bugs like to chill on 'em and you can catch 'em.
Daily Money Tree/Money Rock/Wasp nests - Everyday your island will generate a money hole (Glowing hole you can dig and get 1K bells out of) and one of the 5 rocks on your island will give you money when you strike it. As for the money hole, it's best to do the following when you see one lest you fukk up and lose the ability to re-plant money and maximize your earnings (You know about planting money, right?)
When you locate the glowing spot, immediately take out 10K (You do this by clicking on your money within the nook app and choosing it) and have it on hand (Important) within your inventory. Then, make sure your pockets have room before you dig, because you can risk picking up the sack of coins (1k) and get the "MY pockets are full" message and not be in a position to release a creature to free up a slot if you need be, and would get stuck buying the money again or something. It just throws it off. So yeah, find the glowing hole, have an empty slot available, throw in your 10K bag into the hole, Bam. Money Tree. If you plant 10K, it grows three bags, worth 10K each.
The Money Rock requires a bit of a hack to fully get all what it offers (8 random materials, or 8 different packs of money if it's a money rock). Look up how to dig in animal crossing since I fear my explanations are more misleading/confusing than helpful at times, but basically you're digging two (I always dig three) holes behind you to make sure that when you're hitting said rock, you don't miss, 'cause one miss breaks the cycle and you lose whatever else was supposed to come according to which strike you failed on.
Wasps - I forgot how many wasps spawn in your island a day, but they sell for good money (I save 'em for the bug guy) and are SUPER easy to catch following this ONE simple method. Shake a tree, while standing DIRECTLY infront of it. That way, when the wasps fall and you get the little animation of your character exclaiming and then they charge you, all you got to do is swing the net and pick 'em up easy.
You've been selling fruit, which is good, but be mindful that your native fruit sells for lesser than the other fruit you'll eventually get (As dude said, any of us can give you all the fruit since we've already done this amongst each other and with friends) so it's worthwhile to save your native fruit (Say you got Pears) to sell on other islands rather than your own that don't feature that fruit. I got Pears, my Bro got them booty peaches. Sunday mornings we take our harvest to each other's island and collect bank.
That's pretty much it, excuse all the rambling. The only other tip I have for you is to build workbenches around your island since you should sort of be getting into a groove of going around your entire island fishing/looking for bugs in like a loop (Atleast that's what I do) so I got workbenche and materials scattered all over, so when my shyt breaks, I can just run and build/pick up another. The worse thing to have happen is seeing a rare butterfly infront of you and you have to run away from it to your one work bench and craft something quick. It's mad convenient.
I think I'l lend this here, said so much to explain so little, but atleast you know the logic behind what I'm talking about if you've read in full. Happy playing, man. shyt's a blast.