the buses display the sign 'niterói / charitas' and are operated by the two companies auto viação 1001 and braso lisboa.  both lines follow an identical route - detailed bus itineraries for the two bus lines can be seen here and here. 

alternatively, there is a boat that leaves praça xv every 30 minutes and reaches praia de charitas in 15 minutes.  

praça xv is easily reachable via taxi in 15 minutes, or via several buses routing along copacabana and ipanema, including bus line 354.