Welcome! Ah yup, hearing you. Photoshop can be a very confusing beast. I reckon that after many years of working with it, I still only use 5%.
It depends on how the templates are set up, but [this tutorial might be helpful. If not, this very simple tutorial(http://code.tutsplus.com/tutorials/iphone-and-ipad-design-templates-and-how-to-use-them--mobile-3418) on clipping masks probably will (it's very old, but should still give you the idea).
If you're still stuck, feel free to get back to us. Nothing worse than not being able to progress with a project.