On each help page related to arcpy (e.g. arcpy functions, classes, or geoprocessing tools) the Python syntax for the "thing" is near the bottom. In this case, it says:
ExportToPNG (map_document, out_png, {data_frame}, {df_export_width}, {df_export_height}, {resolution}, {world_file}, {color_mode}, {background_color}, {transparent_color}, {interlaced})
I don't want to be too cryptic, but you need to do some of the work here.