Here is a guide how to create your first GTIN-13 barcode.
It will open a new Barcodes Document window.
(If it doesn't, create one on your own, using the menu command
File > New.)
Add Barcode and select
from the barcodes list.
Now enter the barcode parameters. You should at least fill in a meaningful name plus the content which you should have received from an GTIN-13 barcode authority. You may keep most other parameters as suggested by the defaults.
Now you're ready to output the generated barcode in e.g. PDF format for further purposes.
Here you find the GTIN-13 specifications.