The Code 39 bar code is used for messages containing decimal numbers, upper case letters, and some special characters. See the specification at BARCODE Island for details.
A message can be printed without modification using the Code39JK font, but if you want to add the optional check character, you can use the enclosed Perl script code39.pl, or the Visual Basic script CODE39.VBS. Note that a Code 39 message must begin and end with the character '*'.