Great question! The exact answer to this question really depends on what science fair project your child has chosen. Generally, the data table will have the rows and columns clearly labeled, and will compare a constant at different variables (for example: body temperature at different times of the day). When completing a table of data, here are a few things to keep in mind:
Is there sufficient data to prove (or disprove) the hypothesis?
Have you verified all calculations?
Is the data clearly labeled?
Below, I've included an article that goes into detail about organizing and presenting science fair project data. You may find some helpful information there.