Frame it up with 2x4’s, make sure the frame is level and that the ground is a uniform depth from the top of the frame (you don’t want the concrete to be 6” deep in some places and 2” in others, a nice 4” deep pad is fine for a shed).

Then watch some videos on YouTube and rent some finishing tools. And if the floor it’s not perfectly smooth when you are done, it’s hidden under a bunch of crap anyway.