When PowerPoint opens, there's only one slide in the show. You add the other slides.
The most obvious way to add a new slide is by clicking New Slide, on the Home tab, as the picture shows. There are two ways to use this button.
If you click the top part of the button, where the slide icon is, a new slide is added immediately, beneath the slide that's selected in the Slides
If you click the bottom part of the button, you get a gallery of layouts
for the slide. You choose a layout, and the slide is inserted with that layout.
If you add a slide without choosing a layout, PowerPoint applies one automatically. You can easily change it, and you'll see how in the practice session.