First Day of a Month

Select 
last_day(add_months(date_column,-x)) + 1 from table_name;


Example:
-- First Day of the current Month.
Select 
last_day(add_months(Sysdate,-1)) + 1 from dual;


-- First Day of Previous Month
Select 
last_day(add_months(Sysdate,-2)) + 1 from dual;