I usually use Photoshop to create my siggy's along with a couple of other programs like Paint Shop Pro, Bryce 3D and Micrografx Designer to create the things that go in my Signatures. However, I usually always put it all together using Photoshop. In order to create a signature or any graphic, you start off my creating a new document and then go from there. Instructions for using photoshop can take up huge volume's of books and tutorials. Basically it's combining portions of, and in sometimes all of several different documents together to create a new document, and it's done by layering the different things together. There's a lot more to it to that, but that's a start. When you look at my signature for example, you would see one picture, but each item on there started off as a seperate document that I saved to my computer under it's own file name. Paul Stanley and the kitties are from different photo's, once I removed them from their background, I saved them as individual Transparent PSD files on the computer. The background I put them on, I created with Bryce 3D, but you can make a background using Photoshop as well, by either drawing it yourself or using some Photoshop filters, or actions. You just have to play around. Every other item on the signature comes from a different document that I layered on the background.
These are the basics. You simply have to play around with Photoshop, until you start to get the feeling of it. The same goes from Paint Shop Pro.
One trick I've learned is when you make your document, make it larger that what you intend the finished product to be. I usely create my signatures at 1100 width by 300 height, pixels, and then when it's finished I downsize it to 550 by 150, which is a good size for signatures. Make sure you flatten your image, once you have it the way you like it. After you've downsized your image, sharpen it. Then add your text if you will be using any. Realize that the text will also be new layers, and when you have your text just right, then flatten the image again, and in Phtoshop, "Save for Web", because that way you can make sure the document or siggy won't be too large in bytes for the message boards, they like it 30 bytes or less. I hope that helps a little bit.