As a scientist, it is your responsibility to provide all evidence to support whatever claims you are making. It isn't up to the judge or other scientists to prove your claim. Without data or results that support your claims, it's not a completed work. Either way, it will show. Pick a topic you like.
Science is found everywhere.
There must be something you enjoy that can be used as part of a science project. A judge considers time and effort as two important factors in a successful project. Judges can usually tell that the amount of effort that goes into your project reflects your motivation. Be crystal-clear in both your written and verbal communication skills. Your ideas should be clearly presented and easy to understand.