a short line of text at the beginning of your book dedicating the book to someone you really like

When do I turn in my dedication?

Around the time you do copy edits. If your editor doesn’t ask for it, you’ll need to bring it up yourself.

Must I do a dedication?

If you really don’t like anyone enough to dedicate your book to them, just skip it. Kidding. It’s definitely optional and many books have no dedication.