If you've never been to London before, I suggest that a short river trip is a great way to see some of the best sites. (Dress warmly. It can be very cold on the Thames in January!).
Follow the signs to the Jubilee Line and take a northbound train for just one stop, to Westminster. As you leave the station, you come out alongside the Houses of Parliament and 'Big Ben'. (Officially, it's St Stephen's Tower. Big Ben is actually the name of the largest bell inside it).
If you want to do some sightseeing before going on the river, you can visit Westminster Abbey which is across the road from the Houses of Parliament. (�8 admission + �3 for the audio guide):
http://www.westminster-abbey.org/
Then go to Westminster Pier. (Go to the bridge alongside Big Ben. Instead of crossing the river, look to your left and you'll see the pier). Here you can board a boat to Tower Pier. (The boats are fairly frequent even in January. Show your Travelcards to get a third off the fares).
The trip to Tower Pier only takes about 40 minutes but you'll get great views of many of London's main historic sites. (The commentary is often quite amusing, as well!).
Tower Pier is next to the Tower of London. You might want to visit it but you should be aware that it's expensive (�15), takes up quite bit of time (2 to 3 hours) and can be rather cold in January:
http://www.hrp.org.uk/
To get back into central London, cross over (actually 'under') the road to Tower Hill underground station. Take a westbound train (on either the Circle or District Line - they go from the same platform) for 6 stops to Embankment.