There is no equivalent to Itadakimasu! in America, so the general rule is that when the hostess raises her fork to begin eating, it is acceptable to commence. If she says, “Please go ahead,” then do so without being urged to do so three times.
At home, some families make it their custom to “say grace” before each meal. They may do this standing behind their chairs before anyone sits down, but it is more common to sit down, then bow their heads while one member of the family says a prayer of thanksgiving. Some families may even hold hands in a circle around the table during the prayer. Even though you may not be a Christian, you can show your respect by bowing your head during the prayer.