If your certificate is for a sealing, they will give you three forms that need to be filled out: a Petition to Seal, an Affidavit in Support of Petition to Seal and an Order to Seal.
These are all fill-in-the-blank forms, asking for information such as your name, court case number, the date you were arrested, the charges involved and what police department arrested you. When you complete the Affidavit form, be sure to read it very carefully and make sure that it is right, because you will have to swear to it when you sign it before a Notary Public.