I am putting down a floor in my basement. I understand that I must first lay out the 8ft X 4in X 2in on top?

the thickness of the plywood really depends on how far apart you will have your 2 x 4 studs..... I would would put about six nails in each 2 x 4 and I would use 3/4 plywood..... you could use less or more depending on how picky you are.....I might even use more.....