Maybe my first post was too vague as you didn't respond so I'll instead tell you how to get started.
Start by writing out a one sentence of what your story is about.
Then write a paragraph, get it written down and firm. If you're an outliner, I'd suggest the snowflake method. (A recent method I experimented with myself and am happy with.)
http://www.advancedfictionwriting.com/art/snowflake.php (Yes, it applies to script writing.)
If you are not an outliner and have no idea how to write a script, go to the sticky at the top of this forum, there are plenty of links to free online professional scripts. Start reading and studying. If you've already done that and know the formatting. Write.
FADE IN:
SCENE LOCATION
ACTION
And get going. If you have never ever written a script the last thing you want is a co-writer. Why? Because you have absolutely nothing to offer. You haven't even gotten your toes wet. After that, dive in. Once you get it written, there's a section of this forum that's lower on the list called SHARE YOUR WORK. Post in 5-10 pages and you'll have ready and willing members of the site ripping it apart and helping it get better. (That's a good thing.)