This is really a preference thing for you. I have a dirt floor where I have put down sand. I LOVE it!! But some people put down linoleum or put a good coat of paint over a wooden floor. Wooden floors however can harbor bacteria and parasites when they get too wet. So use a good floor sealer on wood. For bedding you can use pine wood shavings, (never use cedar wood shavings, bad for the lungs of birds) grass hay, sand or even straw. Bed them deep in either of these.
Most of my coops have a plywood floor (one's concrete).
One of the plywood floors I've covered with HDPE sheet. It's waterproof, tough and the feces doesn't really stick to it. http://www.ebay.com/itm/HD-Polyethy...617?pt=LH_DefaultDomain_0&hash=item20b93bc199
0.40 or 0.60 work. Another advantage is that it can be heat welded so if you need to piece it, you can make it a solid sheet.
It had been covered with a waterproof membrane but it wasn't sturdy and the chickens tore it up.
The only drawback I see with the HDPE is that the chickens won't wear down their claws so they have to get out of the coop or have them trimmed.
There are many types of floors, all with advantages and disadvantages. It depends on your individual preferences and situation.
Dirt floors are nice because they absorb moisture, don't require much cleaning, and tend to stay warm or cool. However, dirt floors also leave the coop more vulnerable to burrowing predators, and can get muddy if water spills.
The floors I use are made of wood. Wood tends to be rather sturdy, as long as it isn't soaked with a ton of water, and gives plenty of predator protection. Wood's main disadvantage is that it is hard to disinfect completely. It simply has too many pores and cracks to reliably destroy all of the bacteria.
Another choice is concrete. Concrete is very durable, easy to clean, and cool in the summertime. However, it is somewhat expensive, and can be hard on birds' feet if not covered in a thick layer of bedding.