Armor is divided into helmets, body armors, and shoes. Getting better pieces will increase various attributes. Helmets control your accuracy and maximum health bonuses. Armor controls your reload speed and maximum health bonuses. Shoes control your running speed and maximum health bonuses.
Melee combat
When wielding a melee weapon, wait for a zombie to get close, then take swing followed by a step back. Create a pattern of swinging and rapidly stepping away to avoid damage.
Money
You will earn a cash bonus for each headshot. Turn off auto-fire to make headshots easier. You also get a standard bonus whenever you use a melee weapon for a kill. Make sure your weapon is upgraded to make this easier.