In London you’ll get to know the Underground (a.k.a. the Tube) very well. Unlike in many cities, staff at each Tube station are usually pretty knowledgeable and happy to advise you how to get from Point A to Point B.
Get an Oystercard for the Tube. You’ll need to “touch in” and “touch out” when both entering and exiting the Tube and buses.
Until you get used to London take care before crossing streets: Remember, cars and buses drive on the left side of the road.