You can obtain firewood by splitting any log with any Axe in the Grid Crafting. Different types of wood will give you different amounts of firewood.
If you want to store your firewood you can place it in the world (in stacks) instead of using valuable slots in your stationary containers. To place the firewood onto a block: hold the firewood in your hand, sneak (shift) and use the right mouse button to place onto the square/space you have identified using your crosshairs. Each time you press the mouse button you will place two firewood onto the block you have selected, which will create or add to a stack. Each block can contain one stack of firewood and each stack of firewood can hold up to 32 pieces of wood. Multiple stacks of firewood can also be placed on top of each other. To remove firewood from the stack, right click on the stack. Firewood is removed two pieces at a time.
Firewood can be used directly in the firepit as fuel, but you can also use firewood to create charcoal.
Refer to the Survival Starter Guide to learn how to do this.