When the first ability resolves, you may look at the top card of your library. If you don't, nothing happens. If you look at it and it's not a creature card, it simply stays hidden. If you look at it and it is a creature card, you may reveal it and put a quest counter on Quest for Ula's Temple, or you may choose to leave it hidden. In all cases, the card remains on top of your library.

If an ability (from Future Sight or Oracle of Mul Daya, for example) causes you to play with the top card of your library revealed, you may still put a quest counter on Quest for Ula's Temple when the first ability resolves if the top card of your library is a creature card.

Although the first ability triggers only on your turn, the second ability may trigger each turn.