First, you need design experience. Having strong people skills is great. You will need to know building codes, construction, and local laws. Most US states allow designers to work on home jobs without a license.
Commercial jobs require certification. When working on homes, certifications and licenses are good to have as they build trust among new clients.
Most states require you to pass the three-part National Council for Interior Design Qualification (NCIDQ) exam. Test applicants need an architecture or design degree and two years of experience.
The American Society of Interior Designers (ASID) has student chapters. If there is one at your school, join it and be an active member.