Whether you've peeled up old carpet and found hardwood, or your existing hardwood is in rough shape, you'll need to decide how, and if, to restore it to its original condition. There are a lot of simple wipe on products available in the flooring aisle to restore the lustre to your floor if it's in decent shape - however badly damaged or worn floors usually need a little more elbow grease to bring them back to life.
The first thing you have to do is decide whether or not the floor can be restored at all. Older floors may have been sanded down a number of times already, and have protruding nails or deeply engrained stains that a reasonable amount of sanding will not remove - that's when you know it's time for a new floor. If there's enough ‘meat' left on the existing floor, sanding the entire floor starting with a 60 or 80 grit paper and working your way up to 120 and eventually 220 should smooth out the surface and remove visible wear prior to applying 3 fresh coats of clear coat.
Consider renting a floor sander - for light duty sanding a vibrating floor sander is best as drum sanders can burn through the floor quickly in the hands of a novice. However, if you have some experience or some patience to learn, a drum sander will speed up the job. Sand the floor until visible scratches and stains are removed, being careful to go with the grain and not scratch the wood surface. When you're happy with the results, get out the vacuum and remove all traces of wood dust before getting out the clear coat.
Get yourself a decent wool applicator with a handle, and carefully apply the first coat going with the grain, and overlapping slightly. Be careful not to paint yourself into a corner - I've seen it happen more than once! Different products have different drying requirements prior to applying the next coat, so check out the instructions on the side of the can before you choose a product (as always!) After each coat has cured, lightly sand the floor with a 220 grit to remove small particles of dust and debris that have taken up residence, and give the floor another good vacuum - you'll notice a remarkable difference in the texture of the floor after sanding each coat, it's a very important step to have satin smooth floors.
After 3 coats of fresh clear coat, the original lustre and resilience of your floor will be back in full force, or you will have breathed new life into an old tired floor - refinishing old oak floors adds a lot of character, and isn't a difficult job if you can clear out of the area for a few days. If you're looking to stain the floor a different colour, you'll need to sand much deeper to completely remove any trace of the original clear coat, stain, embedded wood resins, and any oils that may have been applied to the wood in the past. Refinishing a floor with a new clear coat is a fairly simple process, whereas re-staining an old floor is much more involved, and in some cases can be near impossible!