How does a pilot select the correct ILS when the airport has parallel runways?
The two ILS approaches on parallel runways will have a different ILS Localizer frequency. The pilots will select the correct approach from the charts and then either manually tune in the correct frequency or (for more modern aircraft) select the approach in the flight management computer, which will then automatically tune the correct frequencies. Before actually flying the approach, the pilots will cross-check the morse code identifier either by listening to it or (for more modern aircraft) by reading it off the displays.

Below you can see the ILS charts for the Runway 27L and 27R approaches at London Heathrow (source). I marked the frequencies and the identifiers (as they will be displayed on the screen, the morse code is depicted below).