It seems to me that a set designer would want to take art and theater courses. Of course, I would also think that math would be quite important because of layout considerations.
As the previous response indicated, one gets a degree in architecture, and in such a program, most of the coursework is laid out for you. In high school, math is going to be quite important, and you would do well to get high grades in math. In addition to the above-mentioned coursework, world cultures would be a good area, because architecture, in addition to being a combination of art and engineering, is also a form of culture.