An uneven wooden floor is best levelled with a floor-sanding machine, which can be hired. Ask your hire-shop for clear instructions as to how to use this. Walking about and observing the noise of bare boards will show if there are any gaps between boards and joists. Warped boards may be pulled down by driving nails at angles alternately in dovetail fashion or by pulling down with screws.
The boards are laid across joists and their positions can be seen from the nail locations. If a board is in such bad condition that a section should be replaced, saw across it a short distance from the nails to get the cut to one side of the joist. Lever up at the cut and put a strip of wood underneath to support it. If the wood needs to be replaced past the next joist, try to lever out the nails there. If they will not come, punch them through or split or cut the wood from them. Cut across where the other end is to come. Put battens across the joists to support the ends of the new piece. Trim the ends of the adjoining old board with a chisel if necessary, then cut and fit the new board.
Stairs that creak do so because wood rubs on wood somewhere. Underneath the stairs the ends of the treads may be supported by wedged pieces. Check that these are tight by tapping their ends with a hammer. The treads and risers (upright parts) should have tongued and grooved joints, but there may also be glued triangular blocks in the angles. Failure in these joints may cause creaking. Ask someone to walk on the stairs so that you can observe where the creaking is. Metal repair plates or shelf brackets can be put between offending parts.