Most students start with a subject and get stuck because a subject is not a project. The gap is closed in four steps.
1. Write down the constraint list first. Time until the fair, budget, equipment you can reach, supervision available, and whether you can work outside school hours. Every topic that violates these is not a topic, however interesting it is.
2. Turn the subject into a system. "Marine biology" becomes "the effect of water temperature on the survival of brine shrimp". A system has parts you can change and parts you can measure.
3. Find the variable you can move. If you cannot change anything, you have an observational study — which is legitimate, but you should choose it deliberately rather than by accident.
4. Ask what the result would mean either way. If both possible outcomes are boring, the question is not worth three months. If you cannot predict the outcome at all, that is usually a good sign.
A useful test: describe the project in one sentence of the form "I am measuring X while changing Y, holding Z constant." If you cannot fill in all three blanks, keep narrowing.