in my opinion start off with watching a few tutorials then just get into it - make some flat wall textures that are tileable for example....
try buy:
Next-Gen Texturing Techniques | Eat 3D if u can afford it.
Weapons in particular metals are quite simple to texture actually - u juse need a subtle grey with some low opacity overlays, then on the spec increase opacity of your kind of grungish overlays and increase the speculars..
i'm planning on making some video tutorials about texturing (even though i'm not an absolute pro)
which will cover wood, metal, paint, bricks etc