I'm planning to get my 18-55 Canon kit lens a lens hood to protect it from the elements. Which would be the correct hood: a petal hood or a round one?
You need a round one because the front element will rotate when it zooms/focuses.
The EW-60C seems to be what Canon specs for this lens. That's a round one. Note that a lens hood is designed more to protect from stray light bouncing around inside the lens barrel and/or reducing flare than it is for physical protection from the elements. If you are just trying to provide a bit more protection, be sure to put a skylight or UV filter on the lens.