A good strategy in this game is to kite the walkers. Kiting is when you basically lead the enemy around in circles, it is an effective tactic that keeps the area around you clear of walkers and allows you to slowly work your way through smaller hordes that you come across.
Equip Chem Kit:
Consider having the Chem Kit equipped as doing so can positively impact the resource yields you get from completing missions.
It is important that you try and be as quiet as possible as making a noise can put you in danger. Noises such as footseps and shouting will attract the attention of walkers who will instinctively move towards it's source. If you have to fire your weapon make sure when you shoot it has a Suppressor attached otherwise you can expect to draw a horde of walkers to your location.
The main items that you will be crafting will be lockpicks, bandages, and smoke grenades. To craft them you first need to collect the required materials and then at any time during gameplay bring up the 'Crafting' menu so you can select the item you want to craft. The 'Crafting' menu can also be used to find out how close you are to being able to craft an item if you don't have the necessary materials.
You need to be vigilent as you scavenge as all around you there are traps, these can be anything from bottles hanging from the cieling which will make a noise when touched to bear traps hidden along the path. Learn what to look out for and either avoid the trap altogether or if possible disarm it.
Zombies cannot climb so if you find yourself surrounded by a horde of walkers you neeed to get on top of a car or similar object and try and hang on until help from your team arrives.
The quickest way to bring down a walker is to aim at their head. A headshot should kill them with 1 bullet, sometimes though it may take two.
Unlock More Missions:
The Scout upgrade tree opens up more missions, it also makes survivors more successful and faster at completing them.
Don't attempt to salvage cars that you come across that have a red light inside of them as an alarm will go off as soon as you try to enter them which will bring walkers to your location.
Unlock New Missions:
New missions will automatically unlock on the map when you complete missions. This allows you to choose them before you head into a new game, before you can start the new mission though you will need to unlock new routes to the different areas of the city. This is accomplished by gathering supplies for the camp while completing different parts of the mission. Supplies will be items that you can pick up such as bags and containers.
Make sure you don't remain in one area for too long as it will increase the chances of you being surrounded by roving walkers who will converge on the area. You need to move quickly when looting areas so you don't fall into the trap of drawing the attention of too many walkers.
Your ranged weapons (not including the crossbow) should only be used for killing and ambushing human enemies. You only have a limited amount of silenced shots throughout the mission so you need to make sure you reserve them for taking out humans as quietly and quickly as possible.