It can depend on the theme. Some themes even had an option that let's you specify in the admin panel which side the sidebar will appear on. Almost Spring is one such theme.
Typically you'll find a float: right (or float: left) for the sidebar wrapper. Look for it in the stylesheet (ends in .css) You can try changing it, and see if it works. Depending on how the CSS was written, it may work fine. But be sure to check all browsers, just because it looks right in one, it may not in others.
If that doesn't work, then I would suggest you study up on css a bit, or get specific about which theme in your queries here, or perhaps even contact the theme author.