Ideally, you should save and hire a writer. But I hope you find somebody willing to collaborate. I feel like you're putting a lot of restrictions for this work to be a true collaboration, as you have a set start and ending and you're asking somebody to work with that without pay.
Plus, like @Wocalich said you should add your portfolio to this pitch.
Example Pitch
Title: (Kinda self-explanatory)
Genre: Not just Shonen, add a bit more, is it Fantasy? Comedy? Sci-Fi?
Premise: (A short paragraph giving a superficial overview of the story)
Portfolio: A gallery of your art, to see your level of skill and the style you work on, as a writer should have a good idea about both before writing for you.
Monetization: How will be the split of revenue be between you and the artist? Do you have any previous experience working as an artist that would give a writer any confidence about your project? How realistic are your goals?