Press the Menu Button to enter Blueprint Menu.
Items that are required for crafting can be found in Trash Bins, Closets, Chests, Tool Boxes, Car Trunks and other locations.
Upgrades can be used to increase attributes of weapons - Damage, Durability, and Handling. They can be obtained by helping other survivors in Encounters. Select a weapon in your inventory and then LT to upgrade it.
Blueprints are used to create items and add elemental effects to weapons. You obtain them on missions, in shops, and find them while exploring.
Weapons can be improved by using Upgrades and Blueprints. Upgrades will increase the Weapon Stats, while Blueprints add elemental effects.
Boosters provide special Buffs. They are created using plants that you find in green areas, difficult to reach locations, and underwater. Some are only found at night.
A Few Words on Resources
As odd as this may sound, there really are no worthless resources in the game - even String! Heck, you use String for all sorts of crafts so never fail to collect it!
Most of the low-value resources are the most often used, so when you are scrounging for crafting resources - or shopping at a merchant - it may be in your best interest to buy up all of the low-cost crafts resources that they have because you never know when you are going to need it!