Okay, this is a rough draft and I live in a rural area so I am just theorizing here.

You live in Tokyo Japan in an urban apartment complex. You see on TV scenes of carnage, people being ripped apart by zombies, what do you do?

Stage 1: Spread the word

run door to door in your apartment complex and tell everyone to turn on the news

Stage 2: Prepare

Plug the drain on your bathtub, shower sinks etc and fill them up with water. If you have any containers, fill them up also. Have your neighbors follow suit.
Amongst you and your neighbors consolidate and ration all available food until basic gardens can be planted on the rooftop if your building doesn't already have a community garden and central courtyard.
Find anything that can be used as a weapon and arm yourselves

Stage 2 a: barricade based on your buildings architecture. If you live in a single entrance building with no courtyard, barricade the main entrance with whatever you can find, shut down the elevator and block the stairs on the bottom level.
If your building has multiple entrances and a central courtyard, I would recomend pushing cars, or better still, vans into the hallway entrances and barricading around and underneath them. once again, barricade the stairs.

Stage 3: go to the roof and inspect your surroundings. Are the other buildings taller, shorter or the same height as yours and how far away are they? If they are taller, are there windows that you can break? If they're shorter, are there windows in your building that you can break? This is important to know

Stage 3b: Go around your building and collect any tools that you can find. Use them to remove any fire escapes, metal stairways and any other metal that you can use to construct a bridge. Choose your target building and prepare to board.

Stage 4: send a heavily armed party into the adjacent building to clear out any zeds and barricade, or failing that, contain them to a floor that you are not using.

Stage 5: repeat step 3 and 4 until you have reached the suburbs, open water or have a network of buildings sufficient in size and resources to support your group indefinitely. Only descend to ground level if you absolutely must, or you are armed well enough to make your escape to more easily defensible territory and of course take all standard precautions as far as infection containment, survival practices and human relations are concerned. I got you out of the city, now we must all go are separate ways as we undoubtedly have a massive group at this point and therefore must split up to avoid unwanted attention.
May we meet again, hopefully on the same side.

Like I said before, I don't know squat about the city, so criticize the hell out of this plan so that I can improve it.

Note: If you are living on the streets, you are zombie food unless you escape the city altogether or manage to gain entry to a building. When cornered, look up. There may be a fire escape within reach in which case you can climb up to the roof and tell the other survivors what's what