I'm writing a book about a girl who's a Yandere. I've got everything all figured out, but how do I make the plot interesting?

What sort of book is this? Fantasy? YA dystopia? It affects what advice might be given.

One thing to do is add conflict, whether that's some big external threat (Ah! A Dragon! We've gotta kill it!) or something smaller, but still important that forces your character to change, confront something she's uncomfortable with, or throws her life into turmoil.

With conflict you have to add motivation too. (Why should SHE be the one to kill the dragon? Why does she feel like she is the best for the job? Is she a professional dragon hunter? Or why would she want to? Perhaps a dragon killed her friend and she wants revenge) In real life people look at challenges (like, climbing a mountain) and if they have no motivation to do it then they'll just avoid it (nah, I'll read a book instead). That's a bit different of a situation, but you get the drift, it has to be a conflict your main character is interested in directly (she likes killing dragons) or indirectly (she's interested in keeping her family safe, so she'll kill the dragon) or just forced to confront and overcome (she's kidnapped and forced to fight a dragon in a gladiator ring, in which case her interest would just be survival.)

Basically interesting plots contain three things: conflict, good pacing, and character growth.

On the subject of character growth: What does she learn by overcoming the challenge she's faced with?

A couple things to think about.

If you don't know how to make the plot interesting, you haven't figured out everything yet.

Think on the stories you have read....what made them interesting to you?