How can I start?

It sounds like you've been developing these ideas for a long time now, which is great. The idea you mentioned seems to have a very strong inciting event, so the setting must change a lot at that point and consequently begin the protagonist's character arc. If you're worried about your opening, I think you should just start with something fairly small that introduces the setting. What is your character's home life like? How is their education? Their social life? Are they well-off or in poverty? Many of these things can be revealed to the readers simultaneously through something small like the character perhaps giving a presentation in class, or getting into a fight with someone. It doesn't even have to relate to the actual storyline, though the introductory scene is always a great place to put foreshadowing. But you'll find that you can really entrance your readers and make a great story by showing the contrast between the setting at the beginning of the book and the setting by the end.

The advice that I live by when it comes to starting my own stories is to start as close to the inciting incident as you can. If you start too early, it's going to be boring. If you start too late (which technically can be done), then you'd just have a lot of explaining to do.
In your case, you mention that your MC finds out about a crazy, magical destiny. I would start just before then. That way, readers can get to know a little bit about your character and what her life was like without it dragging on or being too confusing.