You need a "plan document" that contains the terms of the plan, some forms to give employees, and you probably want some literature to give to the employees explaining how to use the plan.
Lawyers and insurance companies are good places to get the documents, as well as companies that are in the business of "administering" these plans (determining how much an employee is entitled to under the plan, helping you fill in the annual Form 5500, etc.).
Good basic question-and-answer info appears in the
Section 125 Q&A column on BenefitsLink, especially the earliest questions.