In our personal experience, the hardest part about a science investigatory project is simply coming up with a good idea. And we suggest that for your investigatory project you find a topic that's both novel and useful.
Check out this science tutorial to learn how to make a bar of soap out of guava leaf extract, which was made by Dragonknight15 and fellow peers for their third-year chemistry class. This guava-infused soap actually helps clear up acne. You'll need sodium hydroxide, guava leaves, containers, and lavender scent.
If you'd rather make liquid soap, we've got another great guide for you to follow: