At restaurants you should wait to be seated by the waiter; don’t just sit at a table if you see one free. In cafes and bars you can sit at any empty table but you must order from a waiter and pay for table service (more expensive than if you order and eat standing up at the bar). It is not acceptable to order from the bar and take it to a table to sit down yourself. In restaurants there is usually a cover or bread charge called either “coperto” or “pane” (and yes you do need to pay for this even if you don’t eat the bread…)
See our page on feeling ripped off for more information about charges for table service.