PowerPoint Online works with some (but not all) of the starter files that are used in desktop versions of PowerPoint.
Built-in Office design templates for presentations
Standard Office design templates for presentations are available in PowerPoint Online by choosing File > New. Click the thumbnail image of a template to open it and get started.
Custom presentation templates
Presentation templates usually come in the .potx file format. PowerPoint Online can't open .potx files, so you need the help of a desktop (Windows or macOS) version of PowerPoint to get started with a .potx file:
Let's say you have a PowerPoint template in .potx format that you want to use in PowerPoint Online. To do that:
In the Windows or Mac version of PowerPoint, open the file and do File > Save As. Change the Type of the file to PowerPoint Presentation (.pptx), and then Save the file.
Store the file in OneDrive or another online storage app.
In PowerPoint Online, use File > Open to open the file and begin working on it.
If you don't have a desktop version of PowerPoint, you won't be able to use your .potx template file as is.
Sample templates online
On Templates.Office.com, there are .pptx sample files that can be opened and worked with in PowerPoint Online.
When you open the web page for this kind of sample file, you'll see an Edit in Browser button (along with the Download button). Click Edit in Browser to open the file in PowerPoint Online and begin customizing it for your purposes.
To find sample files online
Go to Templates.Office.com.
Click the Search button near the top right.
In the Search box, type:
PowerPoint "edit in browser"
... plus a term from the list below that describes the kind of sample you're looking for: