Having some skills in GIMP or Photoshop should do the trick. GIMP is free, PS is not.
If you take some time to experiment with the program and trying out all the tools and such, you'll learn how to use it. At least that's how I learn.
If you're wondering about how to do specific things, look up tutorials for it.
And you know, to start off, go to the File menu in the program and open up a picture you'd like to edit into a banner. Make use of layers. Crop out whatever's unneeded and make it fit in a forum signature.
So yeah, that's all I can help you with. Good luck.