If you gonna make a model where do u get the textures from?
Real photos?
But how do u cut the pictures so they fit on the weapons.

Texture sizes are usualy 64x64/128x128 or 128x512/512x512 (etc etc...), People create their textures in programs like Photoshop or Paintshop Pro, sometimes their from scratch and sometimes their photo based.

Before you even have texture the model should be unwrapped, that's bascily making the 3D shape 2D.
After that you import the 2D image into Photoshop (or...) and "paint" your textures on it or import a picture and edit it so it will fit in.

if you have 3D Studio Max, this video tutorial (http://forums.specialistsmod.net/viewtopic.php?t=3284&start=0) will help you turn a 3D model into a 2D image. ;)

If your using Milkshape3D you can also use the free LithUnWrap program to do UVMapping. I posted some video tutorials a while back.

http://www.dayofdefeat.net/forums/showthread.php?s=&threadid=31869

I would also check out the Zombie tutorial by Psionic.

http://www.psionic3d.co.uk/tutorials.html