The petition or complaint is typically the very first document that is filed in a civil case. It is the document that begins the case, and it is supposed to outline the case that the plaintiff has. That means that the petition names the defendant or defendants and lays out the grievances against those individuals.
The document is supposed to include legal claims and the facts that support those claims, although at the time the complaint is filed, not all facts of the case might be known. The plaintiff can add to the facts and evidence of the case as it proceeds.
The petition usually also includes a section regarding relief. This is where the plaintiff documents what he or she wants the court to order as a remedy to the problems set forth in the petition. The relief is often in the form of a demand for monetary compensation, but it might also require the defendant make some action to correct a situation or publicly acknowledge and apologize for an issue.
The facts included in the petition don't usually need to be the entire story of the case as the plaintiff knows it. Rather, the petition usually acts as a short summary of the plaintiff's knowledge -- either through experience or what they have heard from someone else -- about the case. The petition is then used to notify the defendant or defendants that they are being sued and why.
Following the petition, both sides will continue to file documents in a civil case. These documents will seek to ask and answer questions regarding the facts of the case and might also include motions and summons. Because legal filings must be very detailed and follow certain form, it's important to work with legal professionals who are familiar with civil law processes.
Source: FindLaw, "Starting the Case: Initial Court Papers," accessed June 10, 2016