You have asked a big question - one that is not answered in one paragraph, but with an entire book like "LightWorks in Archicad." You might also review the reference guide, page 28 for material attributes and page 322 for materials. It makes multiple confusion since we have numerous rendering options, each of which needs materials to be specially prepared.
A basic material creation would include:
Start in the Material Dialog by duplicating an existing material.
Adjust the dialog to see the material by the "Internal Engine"
You'll note where a material refers to a texture. You will substitute a new texture image for the texture image already referenced by the material. If the material is a LightWorks material, it could also refer to a shader or both a texture and a shader.
If you want to add your own textures, place those image files in a folder that is loaded with the library. Those images will become accessible through the dialog and will always be available to the material.
You will also see adjustments for color, transparency and reflectance qualities in the material dialog.
If you can get to California the week after next, I am offering a series of one day seminars based in the lessons of my book that will bring you up-to-speed on this and other rendering issues.
Santa Cruz Jan 29
San Francisco Jan 30
Los Angeles Jan 31
Orange Feb 01
Dwight Atkinson