Try out the
Tutorials Section.
If you want to copy, and paste it you need the following.
-Gimp, or Photoshop, Paint
-A brain
You go save a picture by right clicking once you hold the left part of the mouse until you're done underlining what you want.
Save it go to Save As.
Go to Gimp,Photoshop,Paint.
Go to the File>Open Section put the image you want start editing.
Go to File>Create if you want to start from scratch.
Be Aware: You cannot copy another persons textures, you might get banned, without she/he's permission, nor can you edit them.
I suggest you get some help from pro texturers, and you'll be fine.
I won't type any names cause you probably would spam there inbox, I might get in trobule.
You can download one of the 3 by going to:
Gimp.org (free)
Paint.net (free I think but lowest quality)
Photoshop.com (not free)