For example The WC3 Viewer, Magos Model Editor, or MPQ-Extractor.
Just look in the Tool section. I suggest the Viewer. WC3 Viewer:
Just look in the structure view, click at the top on BLP (1) , find the skin you want to texture new (the Units are under "Units" or "Textures".
The press the button "Extract BLP" (2) and save it as a .tga file.
Always save as .tga!!!
Now you can edit the texture with a photo edit program.
The best is of course Adobe Photoshop, but it's expensive.
For beginners I can suggest GIMP, which is free downloadable and has many features tha Photoshop has, too.
Here in the tutorial section the are many tutorials for texturing techniques.
My favourite one is Mr Goblins Tutorial.
So have fun with texturing and don't give up. You will become better and better.
Greets chilla_killa