I agree with givi.., all method in documentation, but I have some ways:
1-u draw one line want to rib.
2-exit sketch, pick on rib icon, and select that line just draw.
3- select surface u want to rib and adjust direction of rib
Remember that rib is Ok when line have direction pull to surface, not pull out!
My email: [email protected] . If you have any problems about catia skill pls mail to me!