Here’s an excellent written tutorial for Cycles, but most of it is the same for Eevee, I think:
http://www.blenderinsight.com/beginners-guide-to-cycles-nodes-the-procedural-way/
There is also a follow-up tutorial for procedural wear and tear where you can learn a lot about procedural shading. And he has some free procedural materials, but after these tutorials you’ll probably just want to create them yourself…
