Hi joe, for basement framing lumber use 2x4 for the walls and top plates. But make sure you use a pressure treated lumber for the bottom plates. To clear things up a bit. Plates are the framing that lays on the
floor and on top of the wall studs. Also makes sure you do not put the walls right up against the foundation. Leave at least 3 inches. If you would like more insulation factor you could always use wider lumber, like 2x6 which would give you R19.. If you need any more help please feel free to write back..
Dominick