Squeaking floors are not always possible to make quiet, but there are different ways to attack different floors.
If it is simply plywood over the joists, as with most modern floors, and you have no floor covering or a rug, you can use special screws with special guides that put the screw just the right depth so that a pre-notched shank can be broken off. This gives you a screw in the floor without a head. One of the brand names for this is Squeak-No-More. They are less effective on more complex floors.
Hardwood can be pulled down from below with sheet metal screws and washers to assure they won't cut through to the upper surface.
Special shims can be put between joists and the wood above, if that is where the movement is.
One secret to fixing squeaking floors is to only work on the walking paths, and let the rest of the floor expand and contract as it wants to. If you screw down the whole floor, the squeaks are more likely to force themselves back with changes in humidity.