Order of post when displaying a category

Hi there,
First i hope you'll understand my question cos my english is sometimes poor.

How can i modify the order of post when i display a category.
I'm managing directories and in fact i want a randomized order exept for partners.
I'm using custom post, custom fields. "partner" is a custom field.

Thanks for your help.

testing website : http://ad867090b6.url-de-test.ws