use current date as default value for a column

Add a default constraint with the GETDATE() function as value.

ALTER TABLE myTable 
  ADD CONSTRAINT CONSTRAINT_NAME
    DEFAULT GETDATE() FOR myColumn

CREATE TABLE Orders(
    O_Id int NOT NULL,
    OrderNo int NOT NULL,
    P_Id int,
    OrderDate date DEFAULT GETDATE() // you can set default constraints while creating the table
)