Diagram the cross section of a leaf
Since you don't specify, I will describe the structure of a dicot leaf here. A leaf typically has some sort of covering on the upper layer, such as a waxy cuticle to prevent rot, or a layer of trichomes (hairs) to help control water loss. or discourage herbivores. Immediately beneath this outer layer there is a layer of epidermis, made of tightly fitted, flattened cells that are usually clear so that sunlight can pass through them.
Beneath the epidermis there is the palisade mesophyll layer, a layer of tightly packed, vertically oriented cells which have many chloroplasts and which do most of the leaves' photosynthesis. Below this is the spongy mesophyll layer, which is more loosely packed with air spaces between the cells. The spongy layer does much of the leaves' water handling. Below the spongy layer is the lower epidermis, which contains stomates, pores which allow the leaf to do gas exchange. The outside of the lower epidermis may be covered by hairs or glands.
The links below will shown you a variety of pictures of leaf cross sections that you can work from to create your own diagram.