Minecraft wiki entry for Superflat_survival, associated text: Bones - Bonemeal for flowers, grass, trees, plants / bone blocks for building material.
Arrows - For your bow. Don't waste them as you can only get them from skeletons (no flint, no fletcher villager, no bartering).
Bow - Ranged weapon - Usually badly damaged. Charge it fully before shooting to maximize damage. Bow durability and arrows are limited.