Well.. first you need to have some sort of program to create the image with. You can use the most basic of programs, MS Paint, or use something much more advanced such as Photoshop or PaintShop Pro. After creating the image, you'll need to save it and upload it. You can use a number of different sites to do that but the most common are..
www.photobucket.comand
www.imageshack.usAfter it's uploaded, copy the URL and go to UserPC to the left of the page just under the forum banner. Go into "Forum profile imformation" and paste it into where it says "signature" with image tags around it like this:
[img]www.yourURL.com[/img]
Hope that helps.
Also. Just a tip; if you're going to use MS Paint to make your signature, don't save it as JPEG. Save it as PNG.