During the Action step, players take turns taking actions, beginning with the starting player, and then proceeding in the order specified in the scenario (Frodo, The Witch-king, Aragorn, Saruman in the Trilogy scenario).
When it’s your turn, you may take one action, or possibly pass. Play continues around until all players have passed consecutively. You will usually have more than one turn per Action step, but you cannot take more than one action per turn.