First, I suggest that you look/search for online tutorials on how to create 3D objects in ACAD. Try browsing here: http://cadtutor.net/acad/general/resource/resource.html
This is a quick one...
1. Make sure the floor has no open ends, meaning, it should be a closed shape ie. a rectangle
2. Type REGION, select all floor objects/lines and Enter to exit the command.
3. Type EXTRUDE, pick the newly created region, input desired height, press Enter twice.