I agree with the previous post about the little things that one can do to help make the country better. I think that if you open any American History textbook, you can find a section on "Duties of Citizenship." This might be a very good place to start on deciding what can be done to make this country better. I think that the idea of making a difference is rooted in the basic concept of citizenship. The framers of the Constitution preserved the basic elements of freedoms and rights, but did so with the understanding that all citizens in the nation had to give something back in order to make it better. Civic virtue and responsibility is a part of the framework of the nation. Perhaps, this lies in the basic idea of voting regularly with an informed decision being made, or in giving to public service on a consistent basis. This might even be something smaller, such as helping in neighborhood cleanup projects. In the end, it is this element of civic virtue and public service that the framers did not make mandatory, but strongly advised it.
In addition to the above editor's comments, you may want to think about famous historical figures or other "greats" who have made a difference to his or her country. For example, think about Martin Luther King and his contributions to society and the civil rights movement. How did he make a lasting difference? Think about Helen Keller and her contributions to the non-hearing and non-speaking world. I just watched a wonderful, inspiring movie, Temple Grandin, a true story about a young girl diagnosed with autism and how she learned to cope with being "different but not less", and was chosen to be one of Time magazine's most influential people of 2010. Imagine yourself being nominated for a future edition of Time magazine's most influential people. What would Time say about you? Be specific and detailed while keeping it relevant and personal to who you are and what you hope to make a difference.
I assume you are looking for help in writing a paragraph with this topic.
First you should brainstorm a list of ways you can make a difference to your country. Because this is only a paragraph, I would suggest you pick a narrow time frame and stay within it. For example, "I will make a difference to my country in the next 5 years by..." or "I will make a difference to my country when I am in my 30s by..."
I also encourage you to keep this personal and relevant - rather than idealistic and therefore unrealistic. While it would be awesome to say, "I will make a difference to my country by solving poverty," it is so huge that it will make your paragraph too impersonal and lofty. If I were you, I would focus on the little things that you can actually accomplish that would really make a difference.