Make a grass sickle and whack one of the big trees outside of town with it.

If you press the menu button over any recipe it'll pull up a list of the ingredients and let you go through to the guide entry for each of them, which will give you an indication of which tool/zone you need to collect that item. Different tools give different materials from the same gathering nodes, and levelling them up will let you gather additional material types, the bubble that pops up when you're looking at a gathering node will tell you which materials you can get from it with your current tool.

Thank you, but I don't know, how to make a grass sickle, I don't have the recipe. Can I buy something, that gets me the bark from the tree?

Thank you, I finally made the grass sickle and gathered the flammable bark. That was not easy.

Any time you don't seem to have a recipe you need, it's usually either something you need to buy in the skill tree, or something that comes as a recipe morph of something else. All the armours for example come as recipe morphs of the basic cloth recipe. The list shows you what a particular recipe can morph into, but sometimes you need to go a couple of levels deep to get something specific., when in doubt just try making one of everything and see what it unlocks,