I can make my images/divs responsive by using percentages.
How can I make the font-size responsive to different screen sizes?
If you want it to change when you hit width milestones (e.g. 768px, 370px, etc.), then use a CSS3 media query. If you want it to be constantly changing, then you could use the same percentage, or em, technique as with the images/divs (i.e. target / context = result), but I would suggest media queries.