Zombie Invasion Tomorrow: Be Ready
Section 1: Shelter
As we've all seen in the movies, news of a zombie invasion usually causes mass panic, and many people try to flee cities. The concept of avoiding densely populated areas is a good one; however the worst thing to do is to get caught in a mob of people trying to escape a city. The average modern building is strong enough to withstand quite a pounding by zombies, even if they gain strength in the transformation. So the key is to hole up. Barricade weak points in the building such as doors or first-floor windows. If at all possible, keep to the second floor and above and barricade the stairwells and elevators. Then simply wait it out until the zombies are thinned out enough that you can leave the city safely.
Section 2: Food and water
Stockpile water as soon as possible. Once there are reports of zombies in the city, do not use tap water. In the average room there are many items that can be used to store water; use them! The roof is key to replenishing your supply from rainwater. As far as food goes, canned food is your best bet. It can last for years. It's best to have a healthy amount reserved in advance, because leaving the building to raid grocery stores - or for any reason - is not advisable. However, it is likely that most of the people in the building will have tried to leave the city, so food can be taken from their refrigerators and pantries.
Section 3: Power
Don't count on it lasting for long. Even if your city is fed by a nuke plant, it is likely that it will shut down sooner rather than later with nobody maintaining it. The only thing that electricity is really needed for is heat, but fires can be lit inside the building for warmth as long as EXTREME CARE is taken. Burning down your hiding spot is not recommended. Conserve batteries, because what you have is all you get, and they might come in handy later. If there are any generators around (which is unlikely in the middle of a city), it's probably more beneficial to save the gasoline for use in vehicles when it comes time to leave the city.
Section 4: Weapons
Remember that ammunition is limited, so do not shoot a zombie unless it is a direct threat. If you don't have a gun, a simple bow-and-arrow can usually be constructed using items found in the average room. If all else fails, a sword or some sort of blunt object can be used as a last resort. Weapons will be most useful when leaving the building.
Section 5: Leaving the building
If at any point it becomes necessary to leave the building, perhaps to gather food or supplies, make sure to go in a group of at least three (all armed, of course). Also, have someone covering you from the building at all times. It is best to leave via a ladder or rope from a window rather than moving the barricades to the first floor. If the trip is going to be more than a few minutes' walk, the group should take a vehicle, because the less time spent outside, the better. Also, a vehicle will give some protection. MOST IMPORTANTLY, if anyone is bitten by a zombie, do NOT under ANY CIRCUMSTANCES let them back into the building.
Section 6: Moving out
After the attack has died down, you and whoever else is left will want to leave the city and move to a preferably unpopulated area. Canada's North is a good bet. If you're worried about getting lost, just bring a GPS unit. Zombie attacks don't disturb satellites, so the system should still be running perfectly. By this time, there should be very few zombies left, but there will probably be lots of car wrecks and other obstacles in your way. If possible, send out a scout on a motorcycle beforehand to determine a clear path out of the city. You'll need to bring all your fuel with you, so gather a bunch of gas cans and get siphoning on any abandoned vehicles you find.