In my senior year of High School, I decided to complete an independent study in robotics. I spent the year building a robot that could drive around and shoot tennis balls. I documented the entire project in a lot more detail in a blog located here, but here's a brief summary of what I accomplished.
I spent the first trimester designing the robot with PTC Creo (now I use SolidWorks for everything). The final design was very similar to the original design, with only a few minor changes, such as a different reloading mechanism.