If you're currently alive:
1. Acquiring skills is critical, and requires experience points (XP). XP are gained by damaging a zombie, killing a zombie, using first aid kits (FAKs) only on other people or zombies, building barricades (after you purchase that skill), and spraypainting graffiti in certain places.
2. Most XP come through using tools, and requires surviving. So, familiarize yourself with the kinds of buildings that have good stuff (defense, offense, FAKs, and other tools), and where not to waste your time. Use your action points (APs) to scout out useful buildings you can enter to gather stuff you can use.
For example: Police stations have guns and ammo for offense, and FLAK JACKETS for defense. Flak jackets absorb damage, making you harder to kill. Hospitals have FAKs. Fire stations have fire axes.
3. Stay indoors when you can. When you're outside, you will be unable to enter a building that has been heavily barricaded. When standing in front of these buildings, the description will say "heavily barricaded" "very heavily barricaded" or "extremely heavily barricaded". Anything else will let you in (Very Strong, Fairly Strong, Lightly, Loosely, Doors Closed, Doors Wide Open). If you have a lot of APs, you may be able to break into a heavily barricaded building.
4. Free running is the first skill you should get, because then you can jump inside barricaded (or other) buildings from the inside of any neighboring building (saving wear and tear, time outside, and APs).
5. When free running, watch out for buildings shaded in gray! These are "ruined" and when you try to enter, you will fall to the street, losing 4-5 HP. Plus, you'll be outside again. Also watch for monuments and carparks- jumping to these will put you back outside as well.
6. After you acquire free running skills, you'll probably want one of the skills that boosts your ability to aim.
7. Don't attack other people, because it's poor etiquette and will also get you on a bounty hunter list.
8. If you are bitten by a zombie, you may see the message that you're infected. You will lose 1HP for every action you take until you die! A FAK will restore HP and can CURE YOUR INFECTION!