My question is: do I need to complete a first incremental backup before proceeding with a "real" incremental backup?
You don't need an initial backup to run an incremental.
A simple copy backup job defined as incremental will run as full if no files/folders are in the destination folder.
Actually I already ran a full backup, how do ???I take this as reference for the incremental one?
The destination is automatically used as base for a new incremental backup, so running the same backup multiple times with same source and destination will use as base (reference) the previous backup.