Number 1 - never run a business with a friend. You might end up loosing the friendship if the business goes South.
Number 2 - I am not sure what your question is... If you are asking about how to learn everything possible about Interior Design, I suggest you enroll in an Interior Design Bachelor Degree that is accredited by CIDA - Council for Interior Design Accreditation. I also suggest that you make sure this program prepares you to take the NCIDq (National Council for Interior Design Qualification) so you can be properly registered as an Interior Designer as many states require that you pass the NCIDq exam in order to register.
I hope this helps!