There are plenty of apps available to help you find your way around London using the Tube, but there aren't nearly as many that do the same job for London's bus network. In part this is because the bus network is so much more comprehensive than the Tube network, but London BusMapper still does a good job of helping your find suitable routes to your destination.
Simply let the app find your current location or enter it manually and then input the destination you need to travel to and it will suggest four different possible routes. There's also a handy 'get me home' button that quickly routes you from your current location to your home. It's all very straightforward to use and automatically adjusts the suggested routes based on whether it's day time or night time.
London BusMapper does a good job of making it as easy as possible to find your way around London by bus.