You can use any of the Adobe Photoshop programs, possibly even Elements although I haven't tried that one. If you don't have access to any of those, you can download and use Gimp for free.
Once you have one of those installed, download this default banner and add/change colors, images, on top of it in layers until you got it like you want it.
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loginDo anything you want to the surface, but don't change the size if you can help it.
Other tips...
Try to 'distress' it some. Use 10-20% layer transparency, and/or 10-20% desaturation. May even wanna use a speckled brush and tear or scratch it a little.
Hit me up anytime if you have any other questions.
Niemand and Overdriven are also good sources to ask about banners.
Big tip. Never, ever combine your layers or flatten image until you're completely satisfied. Even then, you may not want to 'save' it flattened. Flatten to create a .jpg and submit your banner, but save your file as layered so that if you ever want to come back and change something, you don't have to re-work the whole project.