This is actual pretty complex and charges depend on time of travel and zones through which you pass. All the different transports are run by different operators and there is no standard charge across the board. So, working out your fares can get quite involved.
Most visitors only really travel within zones 1 and 2 (aka the central zone) because this is where the majority of the attractions are. However, these zones are charged at higher rates. You can save money by not travelling in peak times.
Download the leaflet (below) to see how the different zones are charged. You can also use the single-fare finder on TfL’s website to work out what a trip would cost.
London Visitor Oyster Card leaflet and map: London-Visitor-Oyster-Card-leaflet-map