On this page we will explain in detail where you can collect raw materials in the world of Animal Crossing New Horizons. We have already discussed how the crafting system works in separate section of our guide.
The vast majority of raw materials can be obtained in sufficient quantity if only a certain number of basic tasks are performed daily.
There are large grey stones on your island - if you hit them with a shovel, up to 8 different materials can drop out of them - mainly clay or iron, but sometimes you can even get some gold. Sometimes you'll get a couple of thousand bells. They won't be useful for crafting, but you may spend them on something.
Up to three wood resources can drop from each tree on the island. Therefore, it's worth harvesting wood regularly - it is most convenient to do this when harvesting fruit. If you have to shake a tree to make it drop some fruits, then it's better to hit it with your axe. Just remember not to cut down a tree by accident (ypu can read more about cutting down trees here).
A separate species are bamboos, which can be found on other islands. They don't drop wood but other bamboo materials which are required to craft particular items.
You can also find more resources on the beach (for example, by buying clams, picking flowers or gathering weeds).
It is also worth remembering that from time to time, especially in the evenings, falling stars will appear in the sky. To get their fragments, you have to look up at the sky (change the camera's perspective) and press A button after you notice one (important - you can't hold any tool then). The star will shine and glow. Next day, you'll find some of it somewhere on the beach.
Remember not to sell everything you find - it's best to leave a reasonable supply of a star, and only then exchange the next star fragments for bells in the shop.