Ever walked into someone's house, or office, taken one look around the place and thought "This looks awful. Who designed it?". So have the rest of us. If you can see what's wrong with interior design, you also probably have an instinct for how it could be improved. Not everyone has that instinct. That's why the job 'interior designer' exists, and if the instinct exists within you it would be a great one to nurture.
As with several other industries, family and friends is the best place to start with this. Remake or remodel rooms in their house for them. Take 'before' and 'after' pictures. Start a business website and social media page and display the photos. If people like what they see, they'll order from you. Your end game in all this is to get a partnership with a property construction firm, where they can offer your bespoke room designs as an attractive incentive to potential buyers.