I studied architecture, so I can give you some insight on the design process.
You can enter interior design field with or without a college education. A college degree will definitely help, especially if you want to work at a large firm. But since this is a design profession, your calling card, your resume, your entry to a job is a portfolio of work. If you study interior design in college, the most important document you will take away is your portfolio, not the degree.
Start now. Designers use any and all tools to express their ideas: drawings, material samples, photographs or computers. Can you draw on paper or on a computer? Do you have a camera and understand photography? Start learning about the thousands of variations of the basic materials (wood, metals, plastics, stone, brick, concrete, fabrics, carpet, paint, etc.) and how they are produced and the relative cost.
Look around you, especially in large public spaces. Take some photographs (if allowed), draw what you see and jot down some notes about the space. Do you like what you see? How does the mixture of color and texture help or detract from the purpose of the space. If you are in a library, what materials, lighting,. colors and textures seem appropriate? Would you use the same design for an airport terminal? How about a bank office or a doctor's office? What about your own home?
Somehow, you have to figure out how to communicate (more pictures than words) your ideas for the interior space and feeling of the space you want to create. To be successful, you must understand how people use space in various setting (home, office, theaters, airports) and how most people will respond to your choice of materials and colors.
Find a professional interior designer you can talk to and ask about their career. Ask how they entered the field. Ask what you should do to be successful.