Enroll in a set design program at any college known for its theater program. Read reviews of the college online, paying special attention to any comments about its technical theater program. Apply for a bachelor's degree in set design, as this provides you with courses in all the areas that you need to learn to be a proficient set designer, such as drafting, designing, lighting and more.
Volunteer to design sets for student productions or for community theater in your area. The best way to learn set design is to garner some real work experience for a chance to work on your own designs.
Ask a local set designer to mentor you. If a designer will spend some time with you, offer feedback on your designs or let you sit in on a production meeting you will learn a great deal about the practical aspects of set design.
Get a job as a production assistant with a film company, or as a personal assistant or construction assistant at a theater company. Get as much hands-on experience with sets as possible. Designers need to be able to clearly create building plans, so building experience is practical and necessary.