Am I to assume that it takes a standard action to initiate the aura, which lasts until you decide it should be over? Or is it on whenever? This is rather important for a first round action queue for a cleric that may or may not also whip out Holy Word or something after some of the party is hit with a fear effect.
Nope, no need to assume. The guys at Paizo put it right in the book for you:
Domains grant one ability at 1st level and a second ability at 8th level, as noted in their description. Unless otherwise noted, these abilities are activated by using a standard action.
Note, the quoted text says "and a second ability at 8th level" but clearly there are more than just two abilities. This is because only the 1st and 8th level abilities are "Supernatural abilities". The rest are all "Spell-like abilities" and follow the rules described elsewhere (page 169 for those interested) for spell-like abilities.