Learn to sew. This can begin at a very early age. Ask a mother or grandmother to teach, or take a class from the local culture center or park district. Sewing is a skill that can be useful in many aspects of life. Most costume designers develop an advanced set of sewing and tailoring skills, and knowing this early will give a little edge.
Pay close attention to costumes or clothing worn by different characters in movies, television programs and plays that are seen. Use everything as a learning experience, analyzing whether certain choices worked for a bad guy in a movie or the lead singer in a music video.
Become familiar with all aspects of entertainment and production. Costume design jobs can be hard to find so it helps to be very involved with everything that may bring opportunity, from a local theater group to an amusement park that may need character costumes.
Learn to draw and paint. A very important part of being a designer is being able to show preliminary designs to those that cannot visualize it. This is done in the form of renderings, or colored sketches. A designer needs to be able to sketch quickly and accurately. Be familiar with the body and how it is drawn.