To create a new template in PowerPoint, you start with one that's already there and then modify it as you choose.
To give yourself the cleanest slate possible, start with the design template that PowerPoint defaults to whenever you click the New button on the Standard toolbar. The picture shows you how the default slide looks with this template.
The template's name is Default Design.pot, and although it has styles applied already and even has color schemes, it uses a very basic design. Its background is white, and its text is black. In other words, it's ripe for livening up by you.
So do you just make your changes directly to the slide that's displayed?
No. You change a special slide called the slide master. It's a kind of working face for the design template. Details about this follow.