If you're in the US, some states require you to have a NCIDQ license in order to call yourself an "interior Designer". In Florida, where our company is based, Interior Designers are required to have a license only for commercial interiors work. Residential work is a free for all. Anyone can declare themselves an "Interior Designer" for residential projects.

It's important to note that a licensed Architect (NCARB registered) can offer both Architecture & Interior Design services for both residential and commercial. Any interest in architecture?

http://www.ncidq.org

I would seriously consider an interior design program at the university level. There is a rigor associated with an Interior Design (or Architecture) degree that prepares you for a holistic design approach in your future career. It's an experience that I completely recommend to anyone serious about a career in Architecture, Interior Design, and Landscape Architecture.

In the mean time, I would research some local Interior Design firms in your area to see if they would be willing to have you come in a spend a day or two shadowing a designer in their office.