In it's simplest form, creating an item just requires the project_id in the path parameters. You will usually want to set at least a name, and one of template_idORstructure (but not both).
You can also set a workflow status_id, a folder_uuid and position to set where in the hierarchy of existing items the new item should be placed (the default is appended to the existing items in the project root folder).
You can optionally set the content for the item too, which is an array of objects for every field you want to set the content for. Each object must contain a uuid (which matches a field uuid from the items template or custom structure) and content, which is where the raw HTML, text or other content should go.