The best way to start any essay is to spend some time brainstorming. In this case, of course follow the specific instructions given by your teacher, but it seems you could use examples from personal experience, observations, history, or the media. If you must use evidence from a publication, for example, you can certainly find magazine articles, news stories, and even documentaries about this subject. Some helpful key words (for an Internet search, for example) might be "Real life stories of paying it forward."
I encourage you to start by making a long list of specific examples of people repaying kindness. Once you have several, you have some organizational decisions to make. As you work toward writing a thesis statement (the main idea of your entire essay), you must decide what you want to say about repaying kindness. Perhaps you want to comment on the effects of repaying kindness, or perhaps you want to comment that this is more common in our society than people might expect.
Whatever you decide on for a thesis statement, your next organizational decision revolves around your support (or examples). You could focus your entire essay on one very strong example, and use each body paragraph to develop the example and show how it supports your thesis. Or, you could highlight a few different examples that all support your argument, using a paragraph to focus on each.
I hope this at least helps you get started. Feel free to post further questions as you go, if you get stuck again, and good luck!