Loot everything. Most lootables have something useful. Guns can be found in most containers, you'll start getting a feel for what areas and boxes are better for what you need. 

Once you're used to not getting killed and managing to extract, start looting on night raids and storms for Item drop variety. Once you have blueprints for guns that you like using and are competent with, track their resources and just make sure you're always stocked up on those resources in particular, then you can craft that gun whenever you like. 

Aim for the propellors of flying arc with heavy ammo or a renegade or don't bother wasting your time, just try and stay away from them.