If your child is in elementary school, start with World Book Kids from INFOhio. Once you are in World Book Kids, click the link at the bottom of the page for Science Projects. From that page, click How to Do a Science Project.
If your child is in middle school or high school, start with Science Online from INFOhio. Once you are in Science Online, mouse over Curriculum Tools, and then click Science Fair Guide. It includes lots of helpful tips for planning a science fair project. Then visit Science Reference Center to get more ideas for your science fair project.