For these kinds of tasks, the best practice is to use script tool parameters in a geoprocessing tool which accepts the input you want. If you're learning Python, and do want to accept command line input, the Python window embedded in ArcGIS probably isn't the best place to start. You should have a python.exe included in the installation, and if you run that from the command line, then you can perform basic python operations like this:var = raw_input("Enter your weight: ")
cheers,Shaun