All you really have to do is download WC3 Viewer, in Tools here on the Hive. If you do not have a painting program (I do not recommend MS Paint or anything similar), GIMP2 is a free painting program similar to Adobe Photoshop (I believe you can also download that in the Tools folder here on the Hive).
Now, what you do is that you run WC3Viewer and find the SKIN (.. I can only tell you about skinning, because I don't do modelling) for a model that you want to change the look of - ie for an easy first task, you could be wanting to make a blue Orc, so you find the Grunt.blp file, and export it as a .tga file to a folder you'll find it in. Then you open up that .tga file using GIMP (or any other painting program you have that supports .tga files) - if you don't know how to use your painting program, you need to find a specific tutorial, I can't possibly explain everything in a short manner, but what you need is the Brush and Eraser. You basically do not need anything else for skinning. Proceed to paint over (possibly in a new layer, so that it's easy to go back if you did something wrong) the Orc so that he'll get the look you want him to. When you are done, if you used layers make sure you Merge Visible Layers (under Image), and save the file. Then go back into your WC3Viewer and preview the skin by opening up the grunt.mdx and Remap Textures (under Current File) - find the Grunt.blp in the list, select it and click the button Change.., find your skin file (I believe you may use the .tga, it's not necessary to convert it yet) and use that. The model should change look so you can see how the changes you have made look on the model. If you are satisfied, convert the .tga file to .blp, and import it to your map (if you need help with that, there is probably a tutorial around) if you want to use it, alternatively attach it to a thread in the Texturing & Surfacing board and ask for some feedback.
That should get you started with skinning. You basically just repeat the process and learn from the feedback you receive on the forums. There's a lot of good tutorials here on the Hive on how to achieve the look you want and so on. You should have a look around.
