This would probably be a theme adjustment.
Look for the styles.css file within the themes/[theme name] directory for the theme you are using, and adjust the font settings there.
If you only have course access, you might add the styles manually to your content, perhaps wrapping the HTML of each page in a DIV and setting its font-family to tahoma. (style="font-family:tahoma;"). You can do this with the visual editor when editing content by selecting all the content in the content editor window, then choosing Tahoma from the font-family menu.