This is not the place to ask for a program which can help you with skins. But go to tools and try something... Wc3 Viewer which allows you to convert BLPs to TGAs.. Then you can start working them with any paint program (Photoshop suggested).
And as for spells... What kind of question is this? LOL... Sounds so silly. The Warcraft 3 World Editor of course which you get with the game. Triggers and working in the Object Editor, still included into the World Editor.