Floor squeaks happen when boards or the subfloor rub against each other or against nails, usually because something has loosened over time. Most squeaks are fixable without tearing up the floor.
Fix from below
If you can access the floor from a basement or crawl space, have someone walk above to locate the squeak. Drive a shim into a gap between the subfloor and joist, or add a bracket — don't force the shim too hard or you'll lift the floor.
Fix from above
On carpeted floors, special breakaway screws drive through the carpet into a joist and snap off below the surface. On hardwood, you can drive a trim-head screw at an angle into the joist and fill the hole, or use a squeak-repair kit.
Find the joists
The fix works best when you fasten the subfloor to a joist, not just into the subfloor. A stud finder or the line of existing nails helps you locate joists before driving fasteners.