Wiegted sum

Hi everyone
I want to build a toolset by using add a script that before I had provided in Pythonwin, this tool overlays some rasters by apply Weighted Sum approach. I found that by command arcpy.getParameterAsText(), we can give flexibility to toolset for introducing parameters. but in case this operation, unlike tools like buffer and clip that introduction of parameters is clear, I don't know how introduce required parameters. can anyone help me about this?
thank you