Always ask where the nearest Tube station to anywhere is, as you can not get lost in London if you know that. I have all my holidays there, and go everywhere by Tube, which costs £25.80 for one whole week. Make sure you know which Zone this is in, as there are about 6 of them, and this Oyster card you will get also covers the buses as well.
You can get free Tube maps off the Internet or at any Tube stations. You should see them lying around everywhere.