The first thing you should do is make a record. Take a copy of the publication that concerns you. If you have been defamed online, take a screenshot or video of the defamatory content. Material can be posted and then taken down online quickly. If you don’t keep a record of the defamatory content, there is no guarantee that you will be able to locate it later.
Your record of the defamatory statement would ideally also include the information published around, or linked to the page or statement of concern. Your lawyer will need to review this record in order to give you advice about the strength of your claim.