Creaking floor boards are caused by a loosening of the nails that hold down the boards. This allows the boards to move slightly against each other when pressure is applied, causing the creak. There’s usually no structural danger to a creaky floor, but it can be annoying. Fixing it is just a matter of pinpointing the loose areas and securing them with wood screws so they no longer move.
Set up a drill with a pilot bit. Drill a pilot hole at each place there's a chalk mark.