A good way to combine the study of natural history, drawing, and biology (form and function) is to make sketches on page after page of the plants, animals, and microbes that you are interested in. This is great practice for honing your identification skills and your sketching skills.
It's always best to sketch from living organisms, but it's not always possible. There are hundreds of excellent resources, especially field guides, to work from.
For the example below, I used a pencil for a light sketch and then finished each with a fine-point pen.
Have fun.
A page of fishes. © Fred Montague