many buses going through ipanema and copacabana reach urca (the destination is easily seen on the front of the bus), but stop about 200 meters away from the actual beach.

bus number 512 (urca x leblon) runs along avenida portugal and stops at the forte do são joão, entrance to the praia de fora beach.     

directly from the botafogo metrô station the bus 513 (botafogo x urca) can be taken reaching urca via avenida pasteur.

this link shows praia de fora on a wikimapia - the link as we present it is best seen using google chrome, as it might not open correctly with other browsers.    

on the map is is also possible to see praia forte rio branco, praia de dentro, and the boca da barra.