Well first of all, what are topics that interest you? (biology, chemistry, engineering, physics, computer science etc. etc.) Then, out of those subject areas, pick something that is of interest to you, such as chemistry- biofuels, pollution, biochemistry-starch synthesis, biology-hereditary traits, allergies, physics-cloud formation, gravity, creating torque, etc etc.
After you have narrowed your project into a general idea, then you can start thinking about what project you would like to work on.