A good science fair project is based upon your own interests.  What do you like to do?  What do you find interesting?  What would you like to learn more about?  Pick a topic that interests you, and then design an experiment to test something on that topic.   Below are links to several webpages that share projects done by others.  These can give you ideas on how to design your own project.   However, be sure to modify the project to make it your own.  Do not redo a project that someone else has already designed.