A good science fair project starts with a good topic. What defines a good topic really depends on the person working on it. The best way to go about find a good topic is to start with things that interest you. The more interesting the project is, the better it will come out in the end.
A good science fair project also has a good hypothesis. A good hypothesis will point the student in a specific direction toward a specific result. If it is done haphazardly with no direction, it can not be considered science, any results would just be an accident. A hypothesis will be in the form of a question such as “If I do A, then B will be the result.” There should always be a specific question and goal of the hypothesis.
The rest of the science fair project should include a good amount of research on the subject. There should also be a procedure used to test the hypothesis, and data collection from the testing. The data should be analyzed, and a conclusion should be drawn.
The point of a science fair project is not to necessarily get the right answer. The purpose is for the student to go through the process of coming up with a hypothesis and using experiments to test the hypothesis, with an attempt to find an answer. If the science fair is being judged, the judges will see the intent and the results whether or not they confirm the hypothesis or not. Whether or not the student can comfortable discuss what they have done will also show the kind of work they put into the project.
It is a good idea for the student to be able to talk about why they chose to do a particular project. A good science fair project completed by a student who was interested in the subject will clearly show in the end.