Automatic Update

hi,

i have:

create table tbl_order
(
id int
,price float
,quantity int
,product nvarchar(50)
,date_of_order smalldatetime
,date_of_change smalldatetime
)

insert into tbl_order (id, price, quantity, product, date_of_order) values (1, 23.21, 2, 'A+B', getdate())
insert into tbl_order (id, price, quantity, product, date_of_order) values (2, 22.21, 2, 'A+B+C', getdate())


and i want to make automatic update:
-if new order is made, i want to update field date_of_change with getdate().

i want to put it in a job with some trigger or something that will recognize instant new insert.

thank you in advance,