~ Agree on a business name that is available in your area and marketable.
~ Decide what form of business you want to have - partnership, LLC, Corporation, etc. You should talk to an accountant and an attorney about this as they have different benefits, particularly tax-wise.
~ Go to a lawyer and have the partnership, LLC, or Corporation papers set up right. The formation documents will decide what happens inthe event that you dissolve the company, the company has excess debts to be taken care of, you disagree on issues, one of you is unable to perform your duties, who should get the other half of the company if one of you wants to bail out or dies, and similar matters.
~ Make an appointment with an accountant to set up some sort of a bookkeeping system that you can handle yourself. Find out what tax obligations you have and how you should handle purchases and whatnot, as well as how to transfer property into the company name, if that is something you desire. Discuss who will own the equipment and how it will be replaced when need be. Set up a schedule for payment of various taxes.
~ Decide how to market your business - consider consulting your small business administration for ideas and for classes. Consider marketing promotions, advertising, and your target customers.
~ Decide on your products and services, including pricing and packages.
~ Set up a professional-looking web page and publicize it.
~ Purchase insurance, as needed - on the studio, equiment, business interruption, and for general liability if desired.
~ Schedule meetings with the lawyer and accountant annually to keep everything in check - it's worth the price.