I've had no experience with Calibre, but I know others think highly of it.
My best recommendation would be for you to purchase and devour, as quickly as possible, Liz Castro's EPub Straight to the Point. She can walk you through creating an opf file step by step. (page 103) It's a simple utf-8 encoded text file located within the oebps folder and referenced by the container.xml file in the meta-inf directory. Let Liz show you the details.
But without a proper opf file, your ebook will NEVER validate.