Do they use Euros or Kroner? And which currency should we take. When booking things on line we see the prices quoted in Kroner.
They use neither the euro nor kroner. The currency is the koruna (aka crown), and you should plan on getting some out of an ATM once you arrive in the Czech Republic. Have fun!
Right now according to oanda.com exchange course is 1 US$= 16.32 czk.
You should ALWAYS plan on using the local currency unless. of course, you don't mind paying a poor exchange then used either Euro or dollars. They will always to be happy to take the money because of the profit they are making on the exchange rate. And, then, again, they give you change in the local currency adding to the confusion about being short changed.