how to display message while not filling a particular text n screen painter

I am displayed a user name and password columns in

screen painter.if i will not fill any one of the two fields

means then a message will be display.

My requirement is if i wont fill the "username field"

means then the cursor should not move to the

"password" field (i.e we should not able to enter passwords)

and a message should be displayed to indicating that

user name is invalid.

Kindly send me the coding for this one immediately.....