Every Keynote presentation begins with a theme. A theme is a predesigned set of elements—placeholder text and images, fonts, layouts, coordinated colors, and so on—for creating a presentation. Placeholders give you an idea of what a presentation created with this theme might look like when you use its text and object styles.
Open Keynote, then click New Document at the bottom of the Open dialog.

If you don’t see a dialog with a New Document button, choose File > New (from the File menu at the top of your screen).
Double-click a theme.
Tip: You can set a preference in Keynote to always open a new presentation in a particular theme.

When the new presentation opens, do any of the following:
To add a slide, click
in the toolbar.
To add text, double-click placeholder text and type your own.
To add an image, drag one from your Mac or a webpage to a placeholder image or anywhere else on the slide.
Click
in the lower-right corner of a placeholder image to replace it with your own.
Choose File > Save, enter a name, choose a location, then click Save.
If iCloud Drive is set up on your Mac, Keynote saves the presentation to iCloud Drive by default. You can change the name of the presentation or change where it's saved at any time.
To play the presentation, click
in the toolbar, then press the arrow keys to advance through the slides.
To end the presentation, press Esc (Escape). For more ways to show a presentation, see Play a presentation on your Mac.
You can set a preference in Keynote to always open a new presentation in a particular theme instead of from the theme chooser.
Choose Keynote > Preferences (from the Keynote menu at the top of your screen).
Click General at the top of the preferences window, then select the “Use theme” button.
The first time you set this preference, the Black theme is selected. If you changed this setting before, the last theme you chose is selected.
Do one of the following:
Use the currently selected theme: Verify that the name of the theme you want to use appears after “Use theme,” then close the preferences window.
Choose a different theme: Click the Change Theme button, click a theme, click Choose, then close the preferences window.
After you set this preference, you can still open a new presentation with a different theme. Hold down the Option key, then choose File > New from Theme Chooser (from the File menu at the top of your screen).