Mountains are usually found on the edges of tectonic plates, plates of Earth that sift around on Earth's surface. When two plates come together, they can sometimes create a "fold" in which the plates both move upward as they push together.
I just looked up a diagram online to show you what it would look like. Personally, I'm a visual learner, and the best way I can explain my thoughts to you is through an illustration.
Hope I helped!! :)