Through Google search for soap scum---you will find lots of good information! I don't know how old you are so I am not sure how complicated you want to get. Some things I saw in a short time ---The Tylex company website (not a site just selling the product). The company gives some good descriptions of scum. Keep looking there is good info there. You also can e-mail the company with questions. Many companies take helping young people with their projects as very serious business. Try your local Culligan water company.
I could give you even more ideas if you need them. My BIGGEST piece of advice is to have lots of fun learning what this Yucky stuff is AND PLEASE BE SURE THE PROJECT YOU TURN IN IS DONE BY YOU!! That is MOST important! I will check back if you need some more ideas.