Find an itch that you want to scratch. I am sure that you don't want to scratch my itch, and I don't want to scratch yours. If you have some vested interest in the project, it will help to motivate you to complete it.
Perhaps something to help with a club or group you are a part of. How about checking with your local college or ACM chapter to see if they have a high school programming contest. How about starting up one of your own. What about an application to manage the stats for the $favorite_sport team that you are interested in. Keeping track of rankings for your local tidleywinks club.