You need to put together a package for your art candidates. The package needs to include:
The game design document (GDD)
Access to whatever you have working at present
Concept art direction (what you're looking for visually as regards to style and tone and subject matter).
Art list (exactly what art you need created). [Edit] And the file format you need the art in.[/edit]
Your desired timeline (which art you need when).
A request for proposal - tell the candidate what details you need from the proposal, and what format you want the proposal in.
Before you provide any of those things, it's usually done to have the candidate sign an NDA. But it's unlikely that anybody will steal your game.
[Edit] The main purpose of the NDA is to preserve confidentiality (so the artist doesn't go telling everybody about your game).
And before the artist makes any graphics for you, you need to execute an agreement and an assignment of copyright (you need the right to use the art that you pay for).[/edit]