Berlin is one of my local airports and cities.
Taxis are easy to get but not cheap. Uber is €40-50 into the city centre, taxi can be more.
Where exactly are you staying? I can recommend the quickest route - regional trains or FEX are often quicker than the S-Bahn.
Buy tickets at the machines one floor down from arrivals, or download the BVG app and buy them there. Remember ALL tickets need to be validated before you travel. Daily tickets only at the beginning of the day.
Bring euros. So many restaurants are cash-only, you never know until you try paying the bill (even after seeing card signs at the door…). ATMs are plentiful though so you can always just get some extra cash out if you need.
No forms that I’m aware of, just your Covid pass, which probably won’t be checked anyway. Remember you need the travel one in NHS app, which sometimes requires extra checks. Start sorting it today.
FFP2 masks compulsory on public transport. You can get these cheaply from any supermarket. I’d say compliance is down to 85-90% after being very strict during the pandemic.