To Prologue or not to prologue
(Brunswick, Ga, USA)
I am writing a YA Adventure (Harry Potterish) There are two worlds, Ours (present day) and medieval Germany with castles and dragons and gnomes and such.
My main character (protagonist) lives in present day. But she finds something in her world that is from the medieval world and must be returned.
The bulk of the story is written about her returning it and what happens during her efforts. However, there is much preparation in this world before she can start her quest.
Should I write a brief prologue about the item found, why and how it came to be in our world and then begin the first chapter with my protagonist and her finding the object?
Editors and agents often advise against prologues, usually because they're too long, provide unnecessary information, or repeat something that is told in the rest of the book.
Based on what you say here, my advice is to skip the prologue AND much of the preparation. In your first chapter, show your protagonist find the object, as that is the incident that makes the story possible.
In film that is called the "inciting incident" and we often borrow that term for novels as well.
Normally you open with the normal world, quickly move to the inciting incident that changes the character's normal world, and which requires her to set a goal to make things right again.
You have all those elements in your story, so you just need to get to them as quickly as possible. It sounds as if the real story, or at least the one with all the dramatic action, happens in the "other" world, so it's important to get to that as quickly as possible.