How to get the staring date of week using the current now() function in appian?

now()-weekday(now(),3)

returnType : 3 here is to have first day of week set as Monday and output index to be from 0 to 6. Ref https://docs.appian.com/suite/help/25.1/fnc_date_and_time_weekday.html 

now() - weekday(now()) + 1

You can do it something like that:

a!localVariables(
local!today: todate(local(now())),
todate(
a!subtractDateTime(
startDateTime: local!today,
years: 0,
months: 0,
days: weekday(local!today, 3)
)
)
)