Go on Google and find a camo pattern you like, make sure the picture is only filled with the camo pattern, and make sure its not copyrighted, because you won't be able to use it in photoshop if it is copyrighted.
Once you've got the camo pattern you like, open it in photoshop, i tend to set the image size to 512 by 512, pixels, this way its easily divisible.
to do that go to the top tool bar, image/image size, then adjust.
Once you've done that, you wanna make it a pattern that you can use to blend.
To do that, click on Edit/Define Pattern, then name it something you can relate it to. eg. Camo Pattern 1
Then, when your skinning something and you want it to be camo, select the area that you want to be camo, layer it via copy, then right click on it/blending options
this should bring you to a screen with a whole bunch of options.
on the left side it says Pattern overlay, click the box and then click on the words pattern overlay, the select the pattern you just made from the drop down box, there should be a picture with the pattern you made in there, the set the blend mode to something that looks like its part of the skin, usually overlay works best, then make some adjustments to brightness, contrast etc.
Learn to use the dodge and burn tools as well, it really helps to make things look as if they are part of or imprinted on different textures.
I suggest fooling around with all the tools as well, you never know what you might end up doing.
To make different kinds of textures, try messing around with he filters at the top tool bar.