Young Writer in Need of Help!

Hello, everyone, I’m writing a book on my school computer, but I have no idea what to do to publish it once I’m finished with it. I’m working on the 6th chapter, and it’s on the 103rd page so far. My goal is to have it be a series and I want each book to be about 400 pages or more.

How do I get a book published?

Also, I wanted to know if this is good plot:

My book starts off with my main character, Evi, running away from her twisted and murderous past and into the arms of Jack- the leader of a small band of travelers. Evi thought she got away from people betraying her when she killed Catherine and Henry, but she was wrong. Jack and his crew only use Evi when they believe she has something she wants until Evi finds Blue, an innocent and intelligent spirit, at an abandoned lake house. She also meets Max, a unicorn, and runs away when Jack’s team wants to kill her because they believe her to be the cause of one of their team member’s death.

Evi still doesn’t believe that any of this is real and that she’s just psychotic because of everything she went through.

However, she soon discovers that there are perceptions, like parallel universes. Throughout the first book, Evi will face her past while running faster than ever from everyone in Satré, the magical perception. See, Satré is looking for its next King or Queen. It’s further explained in the book, but it’s basically a giant game where humans are selected by Lords or Ladies of Satré to represent them. Then, they have to kill the other human representatives to make sure they don’t die and they can choose to make their Lord or Lady the King or Queen, or they can opt to become the King or Queen themselves. 

Buuuut anyway, that’s the basics of my book and I was wondering what you think? I have a few different endings in mind, but if I get this published, I don’t want the ending to be spoiled. I’ll just say that what I have in mind is going to make readers really mad until they can get their hands on the second book.

Thank you!

P.S.- If I decide to be a “one book wonder,” then readers are probably going to be very upset with my ending…