The oldest underground railway system in the world, the tube is complicated. There are 11 lines, and these run between 9 zones, 1 and 2 being the most central.
Buy single or return tickets, or get an Oyster card (see London bus for details) in the station.
Taking a few tube journeys in a day? It might be worth getting a travelcard. The aptly named Anytime Day Travelcard for zones 1-6 costs about £15.00. The Off-Peak version costs around half the price and does the same job. But you can only travel after 09.30 on week days.
The tube operates from Monday to Saturday, between around 05.00-01.00. And on Sunday from about 07.00-00.00. There’s likely to be a tube stop near almost anywhere you want to go, so it’s a good way to get around. Never go anywhere without your tube map.