Fiber is a critical resource in the crafting of several items, and luckily you can find it in vast amounts via hemp plants. You can find these hemp plants by exploring both the forest and grassland areas in the game. If you are unsure where these areas are in the game, you can always use the map in the game to find them.
The same also applies to the resources, and you can use the in-game resource locations tab to help you. You can find this tab by hitting m and then clicking it on the left of your screen.
That said, once you locate either the forest and grassland areas in the game, the next thing you will need to do is start exploring and looking for hemp plants.
If you are unsure what the plants look like, they can be easily identified by their purple top flowers.
The plants will also be in groups of other similarly colored plants, so locating them is not hard, and when you do find the hemp plants, all you need to do is hit e on them to gather them.
After hitting e, you will then start to gather fiber, and once you get the fiber, you will be able to use it in crafting later in the game. For instance, fiber can be used to craft the following items such as linen and wooden bows.
However, linen is probably the most important resource that will need fiber, as you need to craft linen to make the best silks in the game, which can then be used to make other items.