The best place to start is a short email: a paragraph about the idea, who it's for, and roughly where you are. We reply within two working days.
None of this is required, but it lets us give you a useful reply, fast.
What the product does and the problem it solves. A paragraph is plenty.
A napkin sketch, an existing app, or a live product that needs a new chapter.
Any deadline that matters, and the rough shape of the budget you have in mind.