After a relaxing, overnight journey by bus from your last European city, check into Baxpax Kreuzberg, one of the more trusted names in Berlin's thriving hostel scene.
Start your day off passing through edgy Görlitzer Park and then, if it's Tuesday, stroll southwards down to the Maybachufer and spend your morning shopping among the colorful stalls at the Turkish Market, maybe partaking in some cheap börek for a late breakfast or hitting up all the vendors offering samples. Toss some Euro-cents into the hat of one of the street musicians before heading on foot along the Landwehrkanal to K-fetisch where you grab a window seat to people-watch in vibrant Neukölln.
You could spend some time wandering around the shops on hip Weserstraße or pick up the U-Bahn at Rathaus Neukölln and head to Mitte, to spend a couple of free, information-dense hours at Topographie des Terrors.
If learning about Germany's recent past proves too much for the early afternoon, duck into the neighboring Martin-Gröpius-Bau and use your current or secretly lapsed student ID-card to get half off whatever hot new show is there at the moment. If you're curious what the fuss is all about, go ahead and take a peek around the corner at Checkpoint Charlie and splurge a little on a hard-to-come-by filter coffee at the hipster oasis Westberlin.
Before its gets dark, take the U8 to Boddinstraße. Grab a slice of pizza from Pazzi X Pizza, an ice cream from Mos Eisely, or a beer from a späti (or all three and not break seven Euro) and head to the glorious Tempelhoferfeld for an unbeatable sunset. If it's Wednesday, you're in luck that drinks at the hole-in-the-wall art-queer locale The Club is offering its 2-for-1 drink special and that it's very close by. If not enough girls show up, head straight away to Silver Future and upon the inevitable drunk hungers, grab a peanut-sauce-soaked sandwich at Sahara.
Split taxi fare with some cute babes you just met who also need to hitch a ride back to Kreuzberg, or if it's a weekend-night, follow a crowd of locals to one of Berlin's many weekend-long parties and, arriving back at the hostel at 5 am or 5 pm the next day, wonder why you paid money for a hostel at all.