Gather all evidence. For example, possible evidence for a vendor who wasn't paid is the original sale contract, proof of any money the vendor received from the nonprofit and emails or letters from the nonprofit regarding the sale.
Talk to people who witnessed the board's actions or have firsthand knowledge. Confirm the witnesses have information that supports the case. Make a list of witnesses.
Go to the official website of the state judiciary system to determine in what court you must file. You usually need to file in the civil trial court, but rules vary by jurisdiction.
Go to the court in person. Bring copies of the evidence and the witness list. Ask for the forms needed to file a compliant — the paper that states your case and starts the lawsuit — a copy of the court rules and a fee schedule. Read the court rules to find out what you must do. Follow the rules and file the complaint and supporting papers to start the case.