Without a certain focus on what you want to achieve next, you will get lost or go mad in the sea of information that is out there. Much is not needed for mundane vehicles and buildings, but there is much to harvest out there even for our simple needs.
A few techniques that come in my mind for starters are the following. Take these terms to Google, see what you get and refine. Bookmark other stuff but try to get one sample into the game and preserve it. Otherwise you are like me, with a harddisk full of cool bits and no output.
Tiling (matching the edges of a texture so you can reuse it without seams showing).
Get familiar with all the materials available in the target game. Try out for yourself when it pays off to have bump map, shine or ambient occlusion.
Invest in GIMP or Photoshop to learn stuff like evening out the lighting of the raw texture. Often one side is brighter than the other or there are some eye-catching features that you want to get rid of. Or you simply want to straighten out the brick rows. A great texture simply applied can show great effects and it performs better than complex materials.
Multiple UV maps let you mix repeated fine details with more blurry overall colour that hides the repetitions. Think of repeated brick pattern plus overall dirt or discolouring layer that is not repeated.
Ambitious people create a high-poly model (which is easier that one would assume if you forget about performance constraints in the game) and bake that onto a low-poly model to produce the texture. Surely not easy, but it frees you from most of the trouble with the 2D exercise of producing a great texture.
One cool place among many is:
https://cgmasters.net/category/free-tut ... s-shaders/This one mostly uses Blender. But don't get put off if they talk about software you don't use. The basic principles behind it are the same all the time.
Very valuable hints focussed on Rail Simulator = Railworks = Train Simulator 20xx:
http://railsimilarity.blogspot.com/http://dereksiddle.blogspot.com/http://the-art-of-rws.blogspot.com/Of course, not all are about texturing, but many deal with materials in one way or the other, and they are all first hand information with the focus on what we do as opposed to monsters, guns and face animation.