How to find start date of month in SQL Server?

SELECT DATEADD(month, DATEDIFF(month, 0, @date), 0) AS StartOfMonth

0 in date functions means 1/1/1900.
The above function first find the difference of total months from 1/1/1900.
It then adds the difference in 0 (1/1/1900) again which will land to the beginning date of month.