This is a great question and very impressive for a 6th grade project. There are many different types of payment in our world and if we add history, there is even more. Let me list a few of them.
First, there is payment in cash. Paper money is by the far the most common in our day and age. Paper money is based on faith in central governments.
Second, there is payment by credit. Banks assess a person's ability to borrow and pay back. Through this, banks offer credit, which people can use to purchase things like homes.
Third, there is payment in precious metals. In the past, this was more common and now there is talk of this again. From what we hear, Iran is taking gold payment for their oil.
Fourth, there is payment based on a barter system. This, of course, is ineffective, but from a historical approach was common.