There are five materials you can find in the game to put together your own items and mods at the crafting tables. The one you'll be using most is in your Base of Operations, but there are some crafting stations in the Dark Zone too. Anyway, the materials are as follows:
Fabric – gear and gear mods.
Weapon parts – weapons and weapon mods.
Tools – weapons and weapon mods.
Electronics – gear and gear mods.
Division tech – Dark Zone blueprints.
Crafting materials have the same rarity scale as weapons and gear. Aside from finding crafting materials from looting (if you pass a building rich in a certain type of material it'll ding on your map) you can also get them from breaking down unwanted items. Destroying a standard item will give you standard crafting materials, and so on.
Blueprints are most easily obtained as rewards for completing side quests, and they have a quality and a level like everything else. You'll have to use crafting materials of the same quality as the blueprint (i.e. standard blueprints will use standard materials).
The level and quality of a blueprint doesn't scale alongside your level. If you have a blueprint for a level six specialised quality holster then it will always produce a level six specialised quality holster, but the other stats and attributes will be randomised every time you create one. This means that a) part of crafting good gear in The Division relies on your luck, and b) as you level up your existing blueprints are going to become less and less useful, until you find new ones.
Given all this it's probably best to avoid crafting until you're further into the game. Early level agents aren't really going to be able to craft anything better than the loot they can find, and early level crafting uses up the materials you could save for more valuable crafting at higher levels.
There are also Dark Zone blueprints, which can only be found in the Dark Zone. These generally produce higher quality, better items than regular blueprints. Dark Zone blueprint items are crafted with Division tech.
If you find yourself in need of higher level materials you can use the material conversion option from the crafting menu to turn lower quality materials into higher quality ones. This does use up a lot of the low level stuff, so if you're going to end up doing a lot of crafting you might want to look at breaking down your unwanted weapons and gear rather than selling them, as it'll produce more crafting materials that you can convert and use. The only material that can't be converted up is Division tech, which you'll just have to keep looting for at your own risk.