You are done with your preliminary research. It is time to work on your variables. Please use this Variables guide before you sit down to write your constants, independent and dependent variables. If you are new to Science Fair Projects, here's a basic guide for you: Variables for Beginners.
Once you identified your variables, it is time for the big event: Writing your Hypothesis. Success and failure of a project depends on many things, but Hypothesis statement is one of the most important part. And from judges perspectives, it is the part that needs to be read carefully.
Google Science Fair Tips: "Explain the variables and how they will be controlled, manipulated and measured."
Google Science Fair Tips: "What’s your hypothesis? Try to address something that you believe is challenging which you are able to answer in a single experiment.
An excellent hypothesis will lead on from the question, be tightly focused and build on existing knowledge."