Pull images from the gallery

Use get_children() (Codex ref):

$images = get_children( array( 
     'post_parent' => $post->ID, 
     'post_type' => 'attachment', 
     'post_mime_type' => 'image', 
     'orderby' => 'menu_order', 
     'order' => 'ASC' 
) );

The first image will be $images[0].

EDIT:

And by “first image”, I mean, the $ID of the first image, which you can use with any of the myriad image- and attachment-handling functions in WordPress. If you can clarify what you want to do with the image, I can provide more specific, further instruction.