Welcome to the Forum Danm:
We don't know how unlevel the floor is but here is a shot at the typical garage floor which would be off 2 to 3 inches back to front.
Using a laser level, mark points in the 4 corners that you will work to, lay out lines on 16" centers on the floor, take a measurement every foot along each line begining with the perimiter pieces, transfer the measurements to 2 by material and rip them out, placing them as you go (use glue and powder actuated nails to anchor, treated lumber.
With all the 2 bys down you can use underlayment grade tounge and groove 3/4" OSB (glued and screwed) as a subfloor and finish with whatever you choose. It will be a long day's work but very rewarding. Let us know how you come out on it.