Sorry, I forgot to add, if you are unfamiliar with London, you will also need a street map. This will show the above-ground locations of all the tube stations in relation to surrounding streets and places of interest. Once you have decided which two stations you wish to travel between, go to your tube map (as shown above) to determine which lines will get you there.
If you are staying in London for any length of time, it is worth getting an Oyster Card (£5 from tube stations, newspaper shops or online), which will give you a large discount on normal journey prices and save a lot of time. You pre-load the card with cash-value and just sweep it over the reader at the tube station barrier at the start and end of your journey.