First off, allow me to apologize for not finding your question sooner.
So, you want to be a fiction novel editor? The first course of action I would recommend is to obtain and master the basics of fictive storytelling. This involves taking high-school and collegiate level courses that focus on creative writing. Try focusing on the short-form first and then move into novels. By focusing your attention on short stories you won't become inundated with all the details a novel entails and instead you'll be able to focus on mastering things such as dialogue, conflict, building a history, characterization, setting and most importantly having a story worth telling.
Practice, practice, practice! Never stop writing and never assume your work is ever good enough. We can always strive to be better. We can always learn something new, even from places we never knew existed.
Next, offer to proofread everything and anything. Look for little mistakes such as misplaced commas, improperly phrased sentences, look for ways to replace adverbs (the words that end in LY) with stronger verbs (action words).
Naturally while a degree in English from an accredited university is always helpful, don't allow yourself to be consumed by fiction. We all love to read and write but a professional editor is a master of the written word but also has skills in social media management, business ethics and communications.
Now lastly, after you've spent years honing your skills, dabbling in your own writing, taking collegiate courses and having mastered the written word, you want to know what being a book editor is like. Imagine playing a game of Dungeons and Dragons. As the editor, you're the Dungeon Master. You know exactly how the story should unfold so it's enjoyable for a wide audience. The author you're working with is the player. They have their own ideas on how the story should go and in most cases, their ideas are relevant and quite good. But, it's your job to guide them, gently, down a road that will allow the story to flourish for a wider audience. This might entail changing characters in the story. Perhaps the protagonist would be better as a woman than a man? Perhaps the age of the character needs to change. Perhaps the setting is too bleak or in retrospect, is too happy. Your job is to find the problems in a work of fiction, what isn't working, what won't appeal to a mass audience and find creative ways to fix said problems without removing the authentic voice of the player (author). Not always an easy task. You will need to be firm yet gentle, understanding but resolute. Patient but stern.
I hope this helped a little. If you have any more questions I'm happy to speak to you more about this topic.
Christopher R. Broom