Either practice drawing/sketching or technical drawing (which is basically using something like an ipad or comp with a pen that you can draw virtual lines with). You will probably be able to choose electives this year or next for art classes for drawing, you can ask your co-ordinater about that. After high-school basically pursue a course that is about game design in which you could ask the uni/tafe about. Basically whatever you wanna do ask someone to help you get there.