Claudio,
Hi. Welcome.
You need to treat this as a straight up business venture.
Despite your broken English I think that your intent and talent came through.
Will you be able to transfer the historical facts / story from the existing Spainish into passable English?
If so -
Try this.
1) Study the craft so you will be able to understand other writers.
YOU need to know what is going on. Any writer you talk with is going to sling around all kinds of terms, rules, guidlines, formats, styles, etc etc and you will need to understand what they mean.
You'll want to feel that you understand the screenwriting process a little bit.
In this process you may deceide to write it yourself.
2) Hang out on several message boards and read posts from the writers.
You want to get an idea how us writers think, what we're trying to do, and get a feel for those you like.
You'll also need to know what the legal arrangments for this would be AND you'll need to create either a payment scenerio or some other type of financial agreement. (shared credits and percentage of money if there is a sale. Just so you know, if you are the IDEA man and the SCREENWRITER does all the WORK THE WRITER gets the bigger number.)
Before you approach a writer you'll need to prepare a synopsis and outline.
You want to prepare something you can give the writer that SELLS THE STORY. Possibly something that captures your vision.
Outline the story. Hit it's high points and detail out where you want it to go. Explain it's theme. Create some characters and outline some of their best moments and how you see them changing over the course of the film.
Then talk to the writers. Be open to questions and ideas.
Focus on what is exciting and marketable about your story and then be willing to listen. You want to be a partner, not a dictator.
Good luck with this and maybe we'll see you around the cooler.