Games can’t handle procedural materials (they’d take too long), so it is common practice to convert a procedural material to an image (the jpegs you’re seeing) to be UV-mapped. Nice and fast.
As to the question:
The only thing you can really do is search around. The blender documentation has an entire chapter each for materials and lighting, plus a buttons reference in the back that explains all the various knobs and doodads in the materials buttons. Look it up and start experimenting.
Search the forum. A lot of times people will post materials they have made. Need something shiny and metalic? Look at the car materials library. These are also excellent candidates to turn into plastics. Need human flesh? There are quite a few ‘skin’ materials here and there. It’s usually pretty easy to tweak one to black, white, yellow, green, whatever. Etc.
Materials have the same basic components no matter what 3D-application you use. Search the net for information on other materials. If you know what specularity, roughness, etc. are, you can usually program a like material in Blender.