Here are some websites that will help you get started:
This online encyclopedia is good for getting a general overview of your topic. You can do a search for your topic, or browse through the many topics on the Organs and Organ Systems page.
Kids Health: How the Body Works
This site lets you click into different parts of the human body and find videos, articles, and more.
Kids Health Articles Page
If you want more targeted information, you can skip directly to the articles page of Kids Health. A keyword search (in the top left of the site) will get you even more specific information.
DK FindOut: Human Body
Even though it won't have information on every topic, I love this website since it has really great visuals.
This database site collects information on topics from different sources. The Human Body page lists several body parts and systems to explore. There is also a Diseases and Disorders page.
After you've tried these sources, you can use your keywords to do Google search for more information. When you do this, make sure you ask yourself if the website looks like one you can trust for good information.