While it’s fairly likely that nobody will check your metro ticket, that’s not a “wink, wink, go ahead” from Germany. The fine for being caught without a ticket is 40 Euros, and if you take the metro enough during your trip there’s a decent chance you’ll get checked. Buy a ticket.
The Berlin metro system can be extremely confusing and the free maps aren’t the best. You might want to print this out (I added pink dots to the stations I thought tourists are most likely to visit).
Don’t confuse Schönefeld and Tegel Airports. They are on opposite sides of Berlin. If you’re choosing between airports and the flights cost the same, Tegel much closer to the center of Berlin, so you’ll most likely save a bit on transportation.
German food is not the best (sorry Germany). Try Turkish food instead! If you don’t eat red meat, you would probably be better off just getting something from a grocery store. In my opinion, the only reason to eat out in Germany would be for the cultural specialties, and most German specialties contain pork while most Turkish ones contain beef.
Germany in April is warm while the sun is out and chilly at night – carry a jacket for when the sun sets even if it seems like t-shirt weather.
ß is pronounced like an “S.”
If you register in advanced you can visit the Reichstag Building, which houses the German Parliament.