More likely than not, when it comes time to form the business you will have partners or investors. It is important to have a proper agreement set in place BEFORE you officially start operating. There are different agreements depending upon which entity the business will be organized. These agreements will address management, business control, partner/investor rights and duties, etc... The more comprehensive these agreements the better. Ask any business owner that has had issues with investors or a partner, they will undoubtedly tell you that their business agreement was one of the most important documents in the business and the subsequent dispute.
The following entities generally utilize the corresponding agreements:
Corporations- Shareholder Agreements
Partnerships- Partnership Agreement
LLCs- Operating Agreement.