You need to use arcpy.GetParameter and set an input parameter as a multi-value parameter.
import arcpy
myList = arcpy.GetParameter(0)
for thing in myList:
arcpy.AddMessage(thing)
arcpy.GetParameter lets you access myList as a list object instead of using arcpy.GetParameterAsText which sets it as a string object and you needing to split it yourself.
Set the input as a multi-value input which will let you add items to a list:
The loop simply prints the values of the myList object, so that's the business section of your script:
I'd also suggest you avoid overiding built-in types like list.