A floor squeaks when two parts move and rub against each other: board against board, subfloor against joist, or a fastener moving in the structure. The repair depends on locating the movement and whether you have access from above, below, or only through the finished floor.
Have someone walk while you locate the movement
Mark the smallest area where the squeak occurs and note whether it lines up with a seam, wall, or joist. A sound that seems a few feet wide may come from one moving connection.
With access below, look for subfloor-to-joist movement
In an accessible basement or crawl space, one person can walk above while another watches the subfloor. A small gap may be corrected with an appropriate fastening method without disturbing the finished surface.
From above, protect the finished surface before fastening
Hardwood, laminate, and carpet require different techniques, and a misplaced fastener can damage flooring or hit hidden utilities. Locate framing and control fastener depth before driving long screws.
Wide movement or sagging is more than a squeak
If the floor flexes noticeably, sags, or started squeaking after a leak, investigate structure and moisture before silencing the noise. Additional screws do not fix deteriorated wood or inadequate support.