Create your own materials. Best way to learn is to fiddle. Think about the whole material first and break it down into sections and work a section at a time.
- Whats the material for
- Whats the colour
- Whats the Specularity like
- Whats the relfection like
- Whats the surface detail like
- Any Subsurface scattering
- Any more surface attributes
Say you want to make realistic stone. Best way is to start with a photograph of the stone you like, and add that as a bitmap in the diffuse channel. Even though you are photo-based you can make the other attributes look as you wish, like the specularity/Bump/Reflection ect, ect.
Here is a couple of basic tutorials to get you going...
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