It won't let you choose Catherine's route since there is currently nothing there. It's just there to hint at a possible route in the future.
Umm, do you mean who you can ask for a date? You can take Anna or Natalie to the beach once you unlock it.
It's mostly up to you at this point. Some events might require the girls to be on a certain level in a certain role, but you'll find those out in the event lists.
