Making a Double Helix model, the structure that most often comes to mind when thinking about DNA, would be a great way to show off what you've been learning in class for your 8th Grade Science Fair project.
You can do this in a lot of different ways, using things like sweets and string, beads and pipe cleaners, or even origami!
If you want to go the extra mile, and do something a little different, another good option would be to do a practical demonstration to show what a strawberry's DNA looks like.
Most people tend to just think about human DNA, and a science fair project is a great way to remind them that living things all around us are made up of the stuff. This is something you could practise first with adult supervision, and then it should go down a treat on the big day!