How would I attach my 6ft wall to my shed? Would I have to nail another 2x4 to all of the borders of the flooring so I can nail my wall on to them instead of the ones that's under my plywood?

Normally you would set the wall on the old floor and nail through the plate into the floor joists. You question is a bit difficult to understand as written. Can you clarify exactly what you want to do. It's easy for you since you can look at the floor but we can't see it.

Is this wall 6' high or 6' long?

A wall is normally built with a plate on the floor and one or two plates at the top. The plate on the bottom is to give support to the S/R bottom edge and a nailer for trim.

Chuck

http://i819.photobucket.com/albums/zz111/lacogada/askmewall_zps8be17cf0.jpg (http://s819.photobucket.com/user/lacogada/media/askmewall_zps8be17cf0.jpg.html)

http://i819.photobucket.com/albums/zz111/lacogada/askmewall2_zpsee3023d4.jpg (http://s819.photobucket.com/user/lacogada/media/askmewall2_zpsee3023d4.jpg.html)