Creating HL2 textures is really really easy once you know how. All you need is photoshop, notepad, the photoshop vtf plugin and some reference photos. Here's some links and tips of my own to help.

Image Editing Programs
Use Photoshop, or if you can't afford it, GIMP. I don't use GIMP personally, and all my experience / tips / tutorial links are for PS. However, Valve's 'Official' (it's a wiki, how can it be official...) Documentation has some info here
There's a vtf plugin for Photoshop available here, and an nVidea Normalmap plugin here. I think there's ones for GIMP as well, but I've never checked.

Reference Picture resource sites:
http://www.cgtextures.com - thousands of high quality photos taken specifically for texture creation, absolutely free! There is a download quota of 15Mb though, meaning you can only download so much in one day, but for the most part you'll only be needing the 'small' class images anyway.

Valve Walkthrough on material creation:
Creating a material tutorial (Valve SDK Documentation)

Photoshop tutorials from CG Textures (Though one of them's for Paint Shop Pro)
CG Textures has a wealth of incredibly useful photoshop tips and tricks, though it assumes basic knowledge of photoshop beforehand. If you're starting from the very beginning

Removing Gradients using the High Pass filter

VTFEdit is another useful tool for making textures, allowing editing of both vmt and vtf files.

Tips:
-There's a slight bug with the vtf plugin for photoshop: if you don't flatten the image (or to be more precise, if you don't have 'Background' layer) before saving it as a vtf, you're alpha channels will be lost! Only applies to see-through things and decals, obviously.
-You can't copy-paste into a layer mask: if you find you need to, you'll have to find a work-around.
-When making decals, I find it easier to work the transparency using a layer mask and then copying that into the alpha channel itself. It's just easier to see what you're doing.

(I'll expand this as necessary. If anyone else has any tips etc to add, I'll include them in the first post)

wow! and i was just about to ask for help regardoing this. but hey, now i don't have to, all the questions i have in mind are answered by you. thanks for this post!


--------------------------
find internet hosting

small addition to that, this also goes for $selfillum which goes of alpha channels (from my understanding atleast) to light a specific part of a texture