I live in a Bi-level, and recently furred the bottom wall where it is concrete and drywalled in my basement. What can I use to make the area level whereIi will be placing my wood shelf on the top off the concrete bottom half of the wall? Is there any filler that can be put down and leveled out before putting the wood down for the ledge?

Any help would be appreciated

One method:

Fasten 2x4 "joists" to the sides of the studs resting on top of the concrete ledge.

Extend them on top of and to the edge of the 'studs' framed in front of the lower concrete ledge.

Then simply mount 1x material or drywall on top of these 'joists'.