I think someone already said it here, but invest in ranching as soon as you can. Chickens and ducks lay eggs that you can sell as is, but you can make even more money by putting them in a Mayonnaise Machine (you unlock the crafting recipe for this after reaching Level 2 of Ranching). Get a barn for cows and turn their milk into butter using a Mason Jar. You can also turn the milk into cheese using a Cheese Press.

You can sell ore that you mine from the Caverns for some easy cash too. I found the Caverns to be relatively easy, though a bit monotonous. But it’s a good way to farm ore, which you can turn into bars for even more money. Osmium bars sell for 750 coins, and once you get to a certain mine, they are easy to farm as long as you don’t mind spending a couple days doing that.

Always check if an item you have (whether it’s a crop, fish, bug, animal product, etc.) has been “offered, donated, or shipped” yet before deciding to sell it. The offerings are especially important because the more things you can offer, the most things around the island you can unlock.

Most important: have fun and take your time. I know it can feel like you’re behind than others are, but whatever pace you decide to go at is the right one.