How to write error message?

Hi,

I have a question about how to write message.

In the selection screen, when user enter a customer code, if the records belong to the customer code has been deleted from internal table T. No data will come out. I need to wite the message when the user enter an uneffected customer code.

Questions:

1,Where can I write the warnning message in my program.

2, How to write it?

Please help, Thank you.