you can start with shells to begin this huge process. I had lots of money when i did this(although i wasted most of it). I suggests that you sell coral, conch, white scallop, scallop, and pearl oyster.
Once you had money, you can start getting letters/paper to write to villagers you have in your town. You can get paper anywhere from Nook, Booker(lost items), or anywhere else you can think of.
You can mail them anything(but trash), like seashells, and write a simple message to them like "How are you?" or anything like that.
if you're lucky, they'll give you fruits. plant them at a spot that they'll grow for sure, and once it had reached its full length, shake the fruits off and plant more of them until you want to sell them.
if you have any friends over WFC that has friend codes, invite them over, or you go to their town, to "exchange fruits they have, if you are lucky to have different town fruits.
if you are lazy, just sell the fruits you have in your town. This process may be slow, though.
sometimes you see coconuts on the beach. plant them near the beach(but not on the beach).
rock money is one of the easiest way to earn money. every day, there is a specific rock that has money when you use a shovel(any) against it. To get best results, do this following. (key: X=player, R=rock, H=hole/building/tree, $=possible money spots.) another money space is on where you are standing.
H H $ $
H X R $
H $ $ $
you can actually get a silver shovel to get more chance using the rock money method. to get this, go to the city between 8pm to 12am. go to the right of the city where there are cones. if you're lucky, the cones will be pushed aside and light will peep out from the hole. go in there, talk to resetti, and he will give you a silver shovel.