If you are working through your project as outlined and without any modifications of your own, the experiment is already defined. So you should be able to use the experiment and background information to write your hypothesis.
I hope this helps and good luck with your project. Please post again if you have more questions.
Do visit the link Norman provided, as it is very helpful for writing a lab report. If you still need help, some examples of hypotheses for this experiment could be:
If the distance between the blood splatter and the source of the splatter increases/decreases, then the size of the blood splatter ellipses will increase/decrease, because....
If the force of impact on the blood source increases/decreases, then the blood splatter patterns will [list some characteristics of the ellipses, such as size or angle], because...