If you want to change the style of the text that appears when you type things that aren't the title or the slide body, do the following:

• Make sure no objects are selected.
• From the Format menu, select Font. 
• Make all the changes that you want there, and click OK. From that point on, new text will be created in that style.

To Set the formatting for the title or slide body objects, go to the Slide Master and format these objects on the master