Home Depot) you can put the screws directly through the carpet & pad into the plywood. You will need to put the screw through the subfloor/plywood and into a joist in order to get the screw to set in something solid.

Or sometimes you can go in your crawlspace and shim under the board (between the plywood subfloor & floor joist) that is squeaking. I would really recommend the first method though, as that brings your floor back down to level, and shimming makes it have permanent higher spots.