If you need to get the first day of the month in TSQL. You can use the built-in DATEADD or DATEFROMPARTS function.
See the following TSQL Queries example.
DECLARE @CurrentDate Date = GetDate()
--First Option
SELECT DATEADD(MONTH, DATEDIFF(MONTH, 0, @CurrentDate), 0) AS FirstDayOfMonth
--Second Option
SELECT DATEFROMPARTS(YEAR(@CurrentDate),MONTH(@CurrentDate),1) AS FirstDayOfMonth