This will be the most asked question regarding AdSense. The answer is not simple actually. Because the site design will be different for every one. But normally there are few positions which will give you maximum clicks.
1. Just above or below the main heading and try to give 300x250 ad-unit (Text Only)
2. Make use of a link unit on the left panel (if you have one, other wise on the right panel), which should be almost on the same level of the main content
3. Place an ad on the content area, which can be an ad-unit you can try an image or text ad unit here.
But main thing is that try to change ad units and placed and check the statistics, until you find the right place for your site. Also remember you should have a good traffic.