You could start off in the Marketplace, have you considered getting your own website? There are a lot of good free hosting sites that may not have the kind of snazzy customization as having an entirely freehand coded site has but will get the job done in a neat and presentable fashion.
Displaying what you've done and are capable of doing is going to be very important. So I would get well lit photographs of your costumes displayed neatly either on a form or person (or both).
Your going to have to take into consideration what your limits are. Such as if you know you can make a high quality costume if it's simpler VS Taking on a very complex order that overwhelms you and isn't the kind of quality you'd like it to be. You may have to accept turning down orders your not comfortable with.
When you open a business your going to be working to please other people, they will be paying you and expecting certain things. They can be demanding with what they want and understandably so. Sometimes it's not always about "I love what I can do!" but "Will I be able to provide for my customers?"
I would also say drafting up a very good order agreement that must be signed before commissions start. Such as having a certain portion of a commission non-refundable (such as the cost of materials) and having some kind of upfront cost that comes from the overall price. Like material cost is upfront. Make a return policy. A policy if someone is not satisfied (within reason). A policy for making payments for the commission over several weeks/months/whatever.
You could try googling 'starting a business' to get some idea's on the matter, obviously a lot of things won't be applying to you but regardless you could get a good idea on what kind of technical stuff to consider.