Most new rotors are covered either with grease, or a wax-like substance similar to cosmolene to keep them from corroding in storage. You must remove this with a solvent such as brake cleaner before putting them into service.
If not, you'll get a squeak.
Also, it's a good idea to both rough up the pads with an agressive paper, like #40 and file a 45° chamfer into the leading edge of the pads with a file.
Follow this, and you'll never have a squeak.