To initiate your lawsuit, you must file a petition or a complaint.
A petition or complaint (different states call them different things) is a short, plain statement sufficient to show that the plaintiff should be entitled to relief. It sets out the facts according to the plaintiff and shows the relief requested. It may express the plaintiff’s legal theories in support of the request for relief.
A plaintiff must have a good faith basis for filing anything in court. The court can sanction you if you file anything in bad faith. You could be ordered to pay a lot of money to the court or the defendant, so be careful. Make sure you have a valid claim.
Many courts have forms you for pro se plaintiffs to adapt for their complaints. There are also examples online.
If you are the defendant, you must answer the complaint filed by the plaintiff within a specified period of time after it is served on you. As to each allegation, you may admit it, deny it, or state that you lack sufficient information to respond.
Read the complaint very carefully, and think carefully before admitting any allegations.