What training should I provide for staff of an internet cafe?

There are two important components here: the café side and the internet side. Since the internet side is pretty much set-up-and-go, the majority of your staff's time will be spent making drinks and snacks and serving customers. You should make sure they can run the point of sale system, can follow the directions to make the drinks and pastries you're serving, and can deal with customers. For the internet side, a basic troubleshooting knowledge will be fine. Make sure the staff know how to turn on and use the computers and who to call if there is something they can't fix. Most 20-somethings will already have some tech skills.