Initial research and choosing a topic
A science fair project is based around an engineering project or a scientific hypothesis. The presentation at the science fair is the result of your project. In between, you will develop a research plan, fill in a science fair application, do your experiments, and then analyze your results.
First, pick a general topic area that interests you. If you have an idea for a project already, then you are a step ahead, If not, do some reading and research to narrow your focus. Check out Student Resources on this site for help finding project ideas.
Once you have an idea for your project, you need to do more detailed research so you can come up with a research plan. This may prompt you to change your topic. For example, if you are in the Senior Division and trying to do new research, then you will want to change your project if you determine that someone else has already done this research project.