Help With Simple Insert, How To Use Primary Key?

Ive added a primary key called ID to my table, now my insert stored procedure dont no longer work.

i want an unique identifier for each row.

heres my stored procedure:

 
CREATE PROCEDURE composeMessage
-- Add the parameters for the stored procedure here
@username varchar(24),
@sender varchar(24),
@date dateTime,
@subject varchar(255),
@message varchar(2500)
 
 
AS
BEGIN
 
insert into Messages(
"Username",
"Sender",
"Date",
"Subject",
"Message"
)
values (
@username,
@sender,
@date,
@subject,
@message
)
END
GO
 
 
 
 
 
 
heres my sqlcreate table:
 
USE [Messenger]
GO
/****** Object: Table [dbo].[Messages] Script Date: 09/12/2006 15:13:52 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
SET ANSI_PADDING ON
GO
CREATE TABLE [dbo].[Messages](
[Username] [varchar](24) COLLATE Latin1_General_CI_AS NOT NULL,
[Sender] [varchar](24) COLLATE Latin1_General_CI_AS NOT NULL,
[Subject] [varchar](255) COLLATE Latin1_General_CI_AS NOT NULL,
[Message] [varchar](2500) COLLATE Latin1_General_CI_AS NOT NULL,
[Date] [datetime] NOT NULL,
[ID] [int] NOT NULL,
CONSTRAINT [PK_Messages] PRIMARY KEY CLUSTERED
(
[ID] ASC
)WITH (IGNORE_DUP_KEY = OFF) ON [PRIMARY]
) ON [PRIMARY]
GO
SET ANSI_PADDING OFF
 
 
 
 
 
 
 
 
 
As primary keycan't be null, what do i put for primary key for my insert to work?
 
hope you understand what i mean?