To me, the very definition of responsive design is that the page layout changes/adapts to your screen size. Simple as that.
Using percentage widths for your main content and sidebar, that’s basically what it does, so I’d say you’re on the right track!
In general though, responsive design has (almost) become synonymous with using media queries. I think the best way to describe that, is saying that’s an extra layer to it all, because you use SPECIFIC screen widths to define SPECIFIC layouts. Like “_if screen is smaller than 320 pixels, then use these CSS rules. If screen width is between 321 and 480 pixels, then use these CSS rules_”, and so on.
So again I’d say you’re definitely heading in the right direction, now it’s time to experiment a little with media queries. Tip: use a viewport resizer to test your stuff, so you won’t have to keep resizing your browser to see the results.