get a pay-as-you-go oyster card with an initial balance of £10-20 if you don't anticipate using the tube more than 2 times a day on average - single trip in zone 1 (where most tourist attractions are) is £1.8;
or get a pay-as-you-go with an initial balance of £5 and overlay a zone 1-2 7-day travelcard (£25.8) onto it if you anticipate using the tube more than 2 times a day in zone 1-2 on average with occasional trips to zone 3-9.
check your balance from time to time and top-up as needed. try to run the balance down to zero by departure to avoid the hassle to claim a refund.
AVOID CASH TICKET AS IT COSTS £4 A SINGLE TRIP!
Have fun in London!
;D