If the concrete floor (cement is a gray powder, one of the constituents of concrete) under the existing shower is in decent shape, shouldn't have to do anything to it. Most showers today with a built-in base are pretty stout by themselves, unless you plan on buying a very inexpensive unit.
I installed a Canadian-made shower a few years ago in a basement, and used composite (recycled) plastic sleepers to set the new shower on. Didn't want to use wood as recommended by the manufacturer because of potential rotting issues, and wanted to raise the base off of the cold concrete floor to make showering a more pleasant experience.