Download GIMP, and go and find a gimp based tutorial site like GIMPTalk to see what you can do with the program. But this belongs in the Art & Graphics help section.
Buy photoshop, you can do anything with it. You can make lots of signatures, then upload them onto photobucket (www.photobucket.com) and copy and paste the url into your signature. ;) Just don't have sigs that are too big cus they are annoying.
There is no set way to make a signiature, but you'll need either GIMP or PS first. And yeah, this needs to eb moved to Graphics help (-Moved-)