The thirty seconds starts as the ability starts resolving and you start lauding. Someone should be ready to time you before you activate the ability.

Lauding means singing the praises of the creature in question. Complimenting it. Telling everyone of its awesomeness, that kind of thing. Don’t stray off-topic. And resist the urge to say “Please clap”—86% of audiences don’t respond well to desperation.

“Only once” means once for that particular creature. If a different copy of Carnival Barker enters the battlefield, or if Carnival Barker leaves the battlefield and returns, it’s a new object and its ability can be activated. Please be considerate of others around you before designing a deck around blinking and activating Carnival Barker three times each turn.

If another player gains control of a Carnival Barker whose ability has been activated, they won’t be able to activate it. That Barker’s all barked out.