I really like finding amphibians and reptiles and photographing them. Up close, when you can see the details, amphibians and reptiles have amazing colors and textures. You will need a close focusing zoom lens or telephoto macro lens to photograph these details. If you plan on photographing small creatures up close on a regular basis purchase a true macro lens, preferably one with a focal length of at least 100 mm.
The best advice I can give someone to improve their natural history photography is to be a good naturalist. The more you know about your subjects the better photographs you will make. In addition, working fast and knowing your equipment is vital.