Well it depends. If you are talking files (not apps) you would go into the "Settings" in the browser and change the download location. However, if the browser doesn't have that option (which does happen) then you would need a different browser for the downloads to go to the card.
This, however, will not work with apps. If you are wanting to download apps directly to the SD card then you first need to make the card "Internal" rather than "External" storage.