one subway-station: "schlesisches tor" (U1), it's in the district 'kreuzberg', in the south-east.
it's a really good starting-point.
graffiti equipment supplier 'overkill' is right around the corner.
see this google-maps-link.
remaining parts of the berlin wall are nearby (just renovate recently), as are huge murals by blu, os gemeos and many more.
good doner kebab is also easy to find in that area, at "bagdad" at the schlesisches tor for example.
"sofia" is a nice place to drink coffee and smoke. link.
bring warm, functional clothes. it's like -5°C, snow & wind atm.
keep an eye on your stuff; people steal a lot in berlin i think.
get a ticket for public transportation for the whole time you're staying.
organize how you get home / to your hostel on your own, before you go out.
people often want to help, but send you in wrong directions because they're stupid.
use this website.
don't let peoples worn, bitter faces drag you down; this city is a bitch in winter.
ah, and alomost forgot -
PARDY.
berlin is f'ing great for party.
check out "lido" for example, or the "d.o.t. club". of course it always depend on who's playing, but the locations are great, and the public amplification both these clubs have is f'ing great.
have a good time!