first of all, you posted this in the wrong forum. second of all, in order to make a skin, get the wc3viewer form the tols section, extract a blp file form which ever unit you want from the treeview. then convert it to a tga or bmp through the wc3 viewer, edit it, then convert it back into a blp. test it, if it works, your done. just make sure its good and worth downloading.
But first of all other than that, we need a decent image editing program. Paint will work, but you won't be satisfied with your results. Find loads on www.download.com
Secondly, you need some skills Practice and get used to your image editing program. If you think your ready, try your first skin or icon.
Some good image editing programs are Photoshop and Paint Shop Pro. Just to be clear you only need one.
But you need more than that to do skins. You need a program that will allow you to access the skins. I use the Warcraft III Image Extractor, but you can use the Warcraft III Viewer if you want to.(both available here in the tools section.)
As for modeling, I don't know about that, so I can't help you. Just look around here in the forums, because that question has been asked before. Also look at wc3campaigns.com in the tutorials section.
Hope this helps. If you have any other questions feel free to ask.
EDIT: I didn't know you posted already. Oh well. Hope this can help you somehow.