Set default text and drawing shape styles
To set default text attributes for the current presentation, add some text using the text tool, choose Format, Font and format it to your desired default font, color, etc.
Put a checkmark next to "Default for new objects", found in the lower right corner of the Font dialog box.
New text you add with the text tool and text that you add to other autoshapes will take on the font defaults you've just set.
To set other fill, line and other non-text formatting defaults for autoshapes, draw one, format to taste, then doubleclick it to bring up the Format Autoshape dialog box.
Put a checkmark next to "Default for new objects" then click OK.
All autoshapes you draw from then on will take on the defaults you've just set, not just autoshapes of the type you most recently drew.
That sets defaults for the current presentation. If you want to set defaults for other presentations you create from now on, see Create a default "blank" presentation with your own defaults