As others have said, get an Oyster card, it's well worth it, and (as far as I can tell) the money you leave on it never disappears.

In addition, go to https://oyster.tfl.gov.uk/oyster/link/0004.do and register the card. You'll have to use a UK postcode, but then you'll be able to check the balance any time you can go online, rather than having to go to a station and hold the card to the reader etc.