Most people here use something like Photoshop, or Paintshop Pro, or The Gimp (which is free).
Image editors take a little time to learn, but there are lots of tutorial sites that will help get you started. If you already have an image editor that you like to use, then you are ready to start skinning. Just try and choose a simple program to start on, as some of them can be a little complex for beginners. Most people start out with "QuickNotes" or "Beatnik Clock" or something like that. Another good way to learn is to take a skin that you like, and look at all the components in a viewer like ACDSee, and see if you can replicate it. There is usually a file to be written too, usually called an ".ini" file, which tells the program where to put all the different bits. Just dive in, and if you get stuck, just come to this board and ask questions, someone will help out. They are mostly helpful friendly people here.
I hope this helps.