You can rent a pole scraper or buy one in in the floor section of your local big box store. This is a two person project. One to pull the carpet and the other to scrap between the carpet and the cement.
If your simply replacing the carpet with new, there is little prep needed. However if your planning on paint or another covering you will need to remove the adhesive. Most rental centers rent scarifiers that will remove the glue on the surface. It works much like a commercial buffing machine you see used when polishing or cleaning floors, However there are special cutting tools that are fastened to the bottom of the machine and as they move around they scrape and remove the glue and any paint that may be present. Its not all that difficult but the machine is heavy,So you again will need two people to move it around.