The most common legal form for a nonprofit organization is to be incorporated. Every state provides a framework for incorporating. The section on nuts and bolts for starting a nonprofit organization has more about this subject.
Forming a corporation involves, among other things, adopting Articles of Incorporation and Bylaws. These documents describe in legal terms what the corporation is for, what it may do, and how it will operate. The nonprofit corporation law of the state where the organization is being formed may have specific requirements or options that need to be considered.
These samples can show how such documents are arranged, what topics they usually cover, and other general features. The actual documents for a particular organization will have to be developed carefully with the plans and resources of that organization in mind.
In general, observers recommend that the Articles of Incorporation and Bylaws be simple and straightforward, specifying only what is absolutely required to put the affairs of the organization in order. More easily updated policy documents of various sorts can be used to cover more wide ranging details and are less likely to be a snag that delays or prevents necessary changes in the future.