File with the appropriate local and/or state government agency.
Depending on legal structure you select, develop and file "Articles of Incorporation, write operating agreement and/or bylaws for use in your internal business operations.
In choosing the legal structure for your business, a number of considerations may affect your decision.
The proposed size of the business is a factor in selecting the legal structure.
If you ARE the business, you have no employees or partners, then sole proprietorship is most likely the legal structure of choice.
If your business will be owned by two or more persons and you will to be a simple business with few regulation requirements, then a partnership may be your choice.
Incorporation is a choice if the business will have enough assets to warrant the cost of incorporation, if you have stockholders needing limited liability; and you can profitably withstand the added governmental regulation and related reporting requirements.
Be sure to get all the information you need for your decision.
The professional advice of an attorney and an accountant is advisable.