I just put a still camera on my bonehead and had the same question. I asked around a lot with camera jumpers on different DZ. This helps a lot and you get lots of input.
You definitely want a wide angle lens if you're just beginning shooting stills. Probably something like a 28 to 24 mm. If you go wider this will have the disadvantage that you have be pretty close to the formation to fill the whole picture.
You don't have to buy a very expensive lens, make sure that it is pretty solid, so it can withstand a couple shocks here and there. You always have sufficient light when you're shooting your pictures, so that should not be a problem.
On my camera (Canon EOS 50) I am working with a 19-28 Voigtlander lens. Most of the times I have it set to 24 mm. By the way if you need to secure your lens setting a tandem rubber band around the lens works great.