How about this:
Survey some friends and family members about your idea and see what they think first.
Volunteer to do a few people's homes for your cost, meaning they only pay for the decor and not for you or your time and using these people as testimonials.
Try to determine how many people in your area are already doing this and if there really is a need for your services.
Once you decide to do it, the best ways to advertise in my opinion are:
listing in the phone book
join the chamber of commerce
network with friends and family and ask for refferals
you could also try leaving flyers or business cards at furniture stores as long as they don't already hire their own Interior Decorators.
Good Luck!