Half Life 2 moved to VPKs a time ago, so you will need to find the hl2_textures VPKs.
They can be found in "steam\steamapps\common\Half-Life 2\hl2"
You will need to open (with GCFScape) hl2_textures_dir.
From there you will see
You can then open up folders and drag textures and materials out.
Source uses 2 (or more) files to create materials, a texture:
.VTF and a material
.VMT. The texture is an image, and the material defines how it looks (is it shiny? does it sound like wood? does it have a normalmap?)
You can read more about it here:
https://developer.valvesoftware.com/wiki/Creating_a_MaterialOnce you've created a texture and material, you need to create a new folder in the Custom directory in "steam\steamapps\common\Half-Life 2\hl2\custom"
Example:
Inside there, you'll create a folder named materials (similar to what you see in the main hl2 folder)
From there you can create a folder (for example: "fluffybutts") and put your material and texture in there.
Note, you'll need to modify the VMT so that it references the folder you created:
You will then need to open Hammer, and create a level to test your texture on; but that's more complex and the
VDC can cover it better than I can.