Questions on German train (bahn.de) from BER > FRA?

I'm going to be in Germany for 6 days at the end of May. I'll be going from Barcelona to Berlin. After 5 days, I am leaving out of Frankfurt back to the US. I've been told the easiest thing to do is take the train (bahn.de). I've looked up flights and there aren't many for some reason. ... show more I'm going to be in Germany for 6 days at the end of May. I'll be going from Barcelona to Berlin. After 5 days, I am leaving out of Frankfurt back to the US. I've been told the easiest thing to do is take the train (bahn.de). I've looked up flights and there aren't many for some reason. Also, I do want to take the train in Germany at least once.

It is a 4-hour train ride:
Berlin Hbf > Frankfurt(Main)Hbf (4:09)

Question 1: Do I really need to book this ticket in advance, or is it just better to do when I'm there?

Question 2: from the website, I can't find where in Berlin (Hbf) the train is. I'll be staying in the Charlottenburg - Wilmersdorf district and I need to know how long it will take me in the morning to get to the train station


Question 2b: is it safe to assume that Frankfurt(main) means the destination is Frankfurt airport?

Many thanks.
6 answers 6