Nick, the most-watched TV network among kids ages 2 to 14, will begin an on-air public service campaign Monday featuring some of its stars offering advice on what young people should do when confronted with hostile texts, e-mails or Facebook posts. The advice offered in one ad featuring Ashley Argota of “True Jackson, VP” and Gage Golightly of “The Troop“: Sign off the computer; don’t reply to a hostile messenger; block bullies from access; make a copy of the message to show to an adult you trust. “It’s not tattle-telling,” Golightly says. “It’s standing up for yourself.” The issue has received national attention after the suicides of teenagers who were cyberbullying victims. Miranda Cosgrove and Nathan Kress of Nick’s “ICarly” are also participating in the campaign, which will last for two years, said Marva Smalls, Nick’s executive vice president for public affairs.