Hardwood floors are an overlooked treasure in many older homes. In their restored state they increase the value of your home and add an antique charm. They may be covered by carpet or laminate, and these first have to be removed before any restoration process can begin. However, once that's done, it relatively easy to restore a hardwood back to its former glory.
Clean the floor thoroughly. Start with just water and rag and see how it does. If that's not strong enough, try using a mild detergent. Don't use too much water at a time, as this can damage the floor. White-spirit should be used over heavy grease marks or any painted areas.
Mend any gaps by cutting a strip of wood that matches the flooring to the proper size. Use wood glue on all its edges and then fit it into the gap. Once the glue has dried you can use a block plane to smooth it out.
Fix the loose floorboards by drilling a hole through the board into a joist -- make certain you're hitting the joist -- hammer a nail in and use a nail punch to make sure the head of the nail is below the surface. Use wood putty to make the hole level with the surrounding floor.
Old floor boards had two different designs, tongue and groove and square-edged. For a tongue in groove flooring, run a circular saw down the edges of the damaged board to cut through the tongue and groove. Make sure to use a blade capable of metal cutting in case there are unseen nails. You'll need to cut the tongue off the new board and fit it into place. Nail it into the joists as will a loose floorboard.
For a square-edged flooring pry out the damaged floorboard, fit in the replacement board and nail it down just as above.
Getting rid of stains and accumulated wax means you'll need a sander. Consider how much of the floor will need sanding and if the area is substantial, sand the entire floor so that it will have a uniform look. Keep the pressure on the sander steady, but gentle. Sweep up the dust and mop the floor again with a damp mop or rag.
Seal the floor with a polyurethane sealer. This will help keep later maintenance down to regular sweeping and a polishing two or three times annually.
Be careful not to damage pipes or cables that may be below the floor. Always wear eye protection when working with power tools. Make sure to ventilate any area in which you'll be using polyurethane.