how to create a trigger?

Hello,

I want to create a trigger for the following table :

[img=http://s24.postimg.org/89cniwszl/1234.jpg]

This is my code :

CREATE TABLE INVOICE (
Invoice_id number CONSTRAINT Invoice_id_pk PRIMARY KEY,
Invoice_date datetime NOT NULL DEFAULT CURDATE(),
Invoice_location varchar2(150) NOT NULL,
Invoice_order number CONSTRAINT Invoice_order_fk REFERENCES order_header(Order_id)
);

What would be the syntax for a trigger for the above table?
I would really appreciate any help :)

thnx!