The Scientific Method
Scientists observe everything in the world from the viewpoint of cause and effect. They believe that all events are the result of a collection of other earlier events. Take a split second. Watch something happen, like fireworks exploding, an alarm clock starts ringing, or a leaf twisting once in the breeze. The events that happened before the split second that you watched are the "causes". The single event you saw during that split second is called the "effect". Scientists, in essence, observe how nature works. You can take a look at a detailed overview to see if the scientific method is a good fit for your project.
The Engineering Method
The engineering design process is the set of steps that a designer takes to go from first, identifying a problem or need to, at the end, creating and developing a solution that solves the problem or meets the need. Engineers create new things, such as products, websites, environments, and experiences. You can take a look at a detailed overview to see if the engineering method is a good fit for your project.
This is information to help you with your science fair project. Through a science fair project you can learn what it is like to think like a scientist or engineer, to investigate and experiment in an area of your interest, and to share your results. A science fair project must use the "scientific method" or "engineering process" and not be just a demonstration or comparison. Also make sure that as you plan your project, you are conforming to all rules as outlined by ISEF.
You can check out some recommendations on how to present a project. Your project should also fall into specific category. If you're not sure what category your study falls into, ask a parent or teacher to help.