Brazilians greet each other warmly and often. Three phrases will carry you through your first day in Rio:

Portuguese	English	When
Oi!	Hi!	Any time, with anyone you'd say "hi" to
Olá!	Hello!	A touch more formal — signs, shops, emails
Tudo bem?	How are you? (lit. "all good?")	Right after oi

Times of day have their own greetings: Bom dia (good morning), Boa tarde (good afternoon) and Boa noite (good evening / good night).