Formal:
* Bom dia: Good morning (used from sunrise to noon)
* Boa tarde: Good afternoon (used from noon to sunset)
* Boa noite: Good evening (used from sunset to sunrise)
Informal:
* Oi: This is the most common greeting in informal settings.
* Olá: A more formal version of "oi," but still widely used.
* E aí?: This is a casual, friendly greeting, similar to "What's up?".
Remember: Brazilians tend to be very friendly and warm. Even if you just use "oi," most people will appreciate your attempt to greet them in Portuguese!