Employees and consultants should typically sign Confidential Information and Invention Assignment Agreements. These agreements help accomplish twin related goals: ensure that the company’s secrets remain secret (NDA); and ensure that the ideas and inventions of employees and contractors belong to the company which pays them for their work (IAA).
Everyone working for a company should sign an NDA, including Board members and officers, and at least everyone with a research, development, engineering or design role in the company should sign an IAA.
A form of NDA, complete with explanations of what its various clauses do, can be found here. That form contract is the first in what is intended to become a self-help legal website for start-ups.