Airfare; transportation to and from the Camino; accommodation; food. Click here for an average cost per day.
Day-to-day luggage transfer; sending a suitcase to Santiago for the end.
Water bladder or bottle. Water is perfectly fine over there - you don't need a filtration system or buy bottled water.
Set up an account on Booking.com to help you manage your stays. I'll be booking some places for all of us, but this is the best site for the Camino bookings.
What happens if I can't go on one day? Can I get a bus or taxi? Yes, yes yes.
Here are some recommendations for guidebooks and apps.
This is totally a personal choice.