Many (probably most) interior designers graduate from school ready to take on the design world, but not the business world. Design firms do not typically require newly hired fledgling designers to use much business expertise, but if a designer decides to strike out one's own, it's a whole new ballgame. Running a business can be quite challenging. Taking a course in basic business management or consulting with someone to help manage ones design firm can be of great benefit.
A very useful resource that I've gone back to time after time is Mary Knackstedt's Interior Design Business Handbook: A Complete Guide to Profitability. The topics covered in this well written and very informative book address all aspects of professional practice, including contracts, types of compensation, setting up a well functioning office, marketing, and much more. I refer to it whenever I need a refresher on aspects of running my business. My copy is an older edition, but the content is still very pertinent. The most recent (fourth) edition includes updated sections with information on software tools for interior designers.
Ms. Knackstedt's blog
also features helpful and encouraging information for designers.