Get chipboard flooring, either individual or packs, the boards are about 3'6" by 1' wide. They are tounge and grooved to interlock them. The best thing to use are chipboard screws, about 1 1/4" or 1 1/2" in length, probabally M8 or M10. They sould be able to get screwed straight through into the joists, if not you can drill a small plot hole first.