When you focus your topic, you should 1) ask an interesting question about the topic, and 2) make this question as specific as possible. In most cases, you will make a better science fair project by building it around a focused question.
For example, Michele was a Grade 4 student who had as a general interest "living things". After gathering some information, she became a bit more focused, and decided that her science fair project would be about "birds". She went and gathered some more information, and started to focus her ideas even more by writing out questions about birds. These questions became more and more focused, as you can see:
Michele's last question was very specific -- so specific that she could run a study to find an answer to her question. Her question became the title of her science fair project, and her project described her study and the results that she collected to answer her question. This project was one of the Grade 4 prize winners at an Edmonton Regional Science Fair!