The Underground--it shouldn't be hard to figure out. For us, it was hard. Mr. Umbrella and I were pretty much an exact replica of multiple scenes in the movie Dumb and Dumber. Eventually, we realized that approaching the desk labeled INFORMATION was super helpful. We were directed to buy a season pass, lasting 7 days on an Oyster Card.
Know before you go: www.tfl.gov.uk/oyster