The two primary sources for tracking a certain congressman's votes are The Library of Congress' THOMAS service and GovTrack.us. They each allow you to find out how a congressional representative voted on a bill.
The Library of Congress' THOMAS service is the best to use for tracking how a member of Congress voted on a specific bill. First, go to the THOMAS home page. Next, click where it says "Roll Call Votes" in the center of the page. Then, select which session of Congress you are interested. The most recent is at the top.
One you've selected your session, you can select which roll call vote you're interested in. Then, click "Major Congressional Actions." Click on the final vote count to see a list of congressmen who voted for and against the bill.
Alternatively, you could follow all the votes of a single member of Congress. First, go to the GovTrack.us homepage. Then, click "Members of Congress." Then select the member of Congress you're interested in following by searching or selecting from the list of all members. On the right hand side, you will see the member's votes on recent bills, plus analysis and comparison to other members of Congress.