Caitlin Margaret’s Answer
Zoology is founded in biology as an animal science. If you are in high school, do your best in your science classes, with an emphasis on biology. Learning biology involves understanding the basic concepts of math, physics, and chemistry, so don't neglect those.
In college, think about what you like about zoology and what sort of topics or careers interest you. Zoology is a broad field, but topics that you will likely want to learn about are animal behavior and development, vertebrate vs invertebrate biology, physiology, etc.
If a particular dream of yours is to work at a zoo, there are lots of careers that allow you to do that. Working directly with animals as a zookeeper, veterinarian, or technician will almost always require a college degree in biology, and may require post-graduate degrees in a related field of study. There are also careers in animal nutrition, zoo planning, conservation, and zoological research that may interest you.
One of the best things you can do when looking at zoo careers is to volunteer at your local zoo. It will give you experience into zoo routine, as well as way to contact current zoo professionals and ask questions.