A good pitch should be the foundation of the work to follow.
It should offer a brief explanation of what the proposed story is about – “brief” being no longer than 200 words. (Those words may wind up being repurposed in your story as the lead or thesis.)
It should be written in the same tone and from the same perspective your project will take, even if the storyline changes while you’re writing/reporting it (which it often will).
It should outline your intended interview subjects.
Most importantly, the pitch should express why you believe the story is worth writing.
You should be able to make a verbal pitch just as clearly as the one you’ve put in writing.