I recently spent more time than I care to think about trying to get a handle on sketchup, and I also had no CAD experience. From engineering pals of mine, I understand that it's a lot easier than true CAD, but the simplicity and debatably "intuitive interface" do have their drawbacks.
Anyway, if you use the arc tool, you can make an arch as steep as a half circle (half as tall as it is wide). Any taller than that it starts to bulge more than an arch would. If you want something with more height relative to its width, the best I've found is that you can create a steep pointed arch by making an a-frame with the arc tool, and just having each side slightly bulge. I hope that helps, and I have found the online tutorials offered can be helpful if you have the patience to sit through them.