hit google and do a search for "Grass PS" and start there. Learning the natural elements like dirt, grass, sand, etc. is much easier than trying to create the side of a building. And most structures have similar characteristics to the basic elements, so your original images can be adapted to new materials. Once you get a fairly large stable of your "Base" images, it's pretty easy to manipulate them into whatever you desire. Some people do one off textures everytime, but i usually dip back into my older textures, because often then yeild good results without having to tediously tweak every aspect.
This is just the way i do things, and the way i learned. there are countless other ways as well.
lee