I think it is GREAT that you want to be an engineer in the U.S. marines! This is a very admirable career ambition.
I would focus on mathematics classes especially trigonometry and coordinate geometry which is heavily used in maritime engineering. I would also recommend strongly another language. Marines travel around the world and it would be great to learn other cultures and languages. It will make you that much more valuable as a marine. Also, I would pay attention to classes dealing with world history and American history. As a part of our armed forces, you will need to understand clearly the heritage of this country as well as our enemies and allies.
Good luck, Shelbie and thank you for your service to the United States of America from one grateful American!
You definately seem to have one of the key ingredients - Motivation. As you go through middle school and continue through high school, try to take as many math and science courses as you can get your hands on. I also suggest trying to join a robotics team (or building/programming your own robot with kits such as the LegoMindstorms or the Vex robot starter kits).