Advice to students for science fair projects
Ask a question and think about what you think the answer might be
Don't ask a question that can be answered by yes or no. You won't learn much. Ask a question that makes you test ideas and make measurements.
Keep the experiment simple. Measure one change at a time. Have a "control" in the experiment--that is, run the experiment without changing anything to get a "normal" result. Then you can compare changes against the normal.
You may test several or many variables, but one at a time. This will mean a lot of trials, but they will help you to understand what is actually causing changes in the outcomes.
Write down all your measurements. If they don't look like what you expected, test again. If they still don't look like what you expected, try to figure out why that happened. You may learn something interesting and important by doing that.
Judges like to see your data. If you have a "notebook" where you kept track of measurements during the experiment, bring it along to the fair. Make tables where you present your measurements, or charts, or graphs that summarize the trials and results. If you have made multiple trials of the same variation (and you should), try to also show the average results. The average is likely to be a more representative result than any individual one.
Put labels on your charts and graphs so we can see what is being measured.
The judge will want to have a short conversation with you. You don't have to make a report. Be prepared to explain your project, to explain how you got the idea, and to show what your results are. Try to do some reading about the subject so you know more than the experiment shows.