Turn your hobby into a science fair project
Starting work from scratch on a science fair project can be a daunting task. So instead start with something familiar that you enjoy like your hobby. Now you have a good starting point for your science project. No matter what your hobby is it can be the base of an interesting science project.
In case you enjoy gardening you can develop a science project based on growth of two seedlings. Take three seedlings of the same kind and plant them in three different kinds of soil. Now regularly monitor their growth and record the height of the seedlings in detail each week.
Observe which soil makes the plant grow best. Now analyze the chemical composition of the soil and figure out what part of the soil helps the plant. Of course this is the kind of project that will take a couple of months to do. So make sure that you start out the project way before the science fair entry is due.
If you are looking for something related to electronics you can take your pick from any number of interesting science projects. Do you like music? You can build a simple radio. There are science fair project kits which will give you all the know how and the materials that you need to do so.