I'm going to stay 2 nights in Berlin. Since I have no experience with German language neither with subways, I'm afraid to be lost. Also (believe me, is true) I don't have a smart-phone (with Google maps, Internet and so on).
I'll arrive at Berlin HBF and my hotel is near Deutsche Oper.
So, I'd like to buy a ticket to use in Berlin for the whole day or maybe for 2 or 3 days.
In this way I don't need to worry if I take the wrong subway or train or bus.
I'd like to use this ticket to go (if possible) from anywhere to anywhere (in the A or B area, if I well understood the notion of areas in Berlin).
Is there this kind of ticket? How it is called? Can I buy it at HBF?
And the most important: should I validate any time I enter in the transport or only the first time?