
Generally for a science fair you are demonstrating something or experimenting to answer questions and showing varying results and predictions.
I would suggest that you think about (brainstorm) the whole project and come up with several different ideas of what it is that you are demonstrating or experimenting with.
For example, when I was in 9th grade I experimented with mice and their diet--one was fed a commercial feed and the other was fed somewhat madeup rations. I did this over a period of a couple of months and recorded their differences in size and appearance and behavior as time went on. If I was re-doing this experiment today, I would try to research the specific nutrients needed and see if I could make a better feed, and how that feed compared.
A few years earlier, I built a homemade incubator and tried to hatch chicks. Either the eggs were infertile, or I did something really wrong and killed them early on. I researched the information on hatching chicks (as a city child, I knew nothing about it, although I got some help from my dad, who helped take care of the chickens for him mama when he was a youngster. I had charts depicting what was supposed to happen, and chick development, etc. It was a fairly good demonstration--except that the eggs never hatched. Someone else had almost an identical project, but her eggs hatched--she placed much better than I did.
So, to get you started, do you want to experiment or demonstrate?
What questions would you like to answer (experiment) or what knowledge have you researched and want to demonstrate?
Do you want to predict something about the cross you're proposing? What?
Do you want to demonstrate something? What?
Come up with some ideas (I have several, but it's your project
)