To begin with, you will need a bachelor's degree, preferably with a focus on interior design (you'll want to have some architecture, drawing, and computer-aided design classes under your belt before you graduate).
To become licensed, you will need to pass an exam administered by the NCIDQ (National Council for Interior Design Qualification). Not to be confused with ICUNDQ, which is text-speak for "I see you in Dairy Queen." You can't even take the exam until you have obtained your design degree, and in some states until you have racked up two years of real-world design experience.
While it isn't a requirement, it's also a good idea to become a member of ASID (American Society of Interior Designers), which will connect you with others in your field, keep you in the loop on current trends and innovations, and give you yet another card to find a place for in your wallet.