Formal:
* Olá, Sabrina. (This is the most common and neutral way to greet someone.)
* Bom dia, Sabrina. (Good morning) - Use this in the morning.
* Boa tarde, Sabrina. (Good afternoon) - Use this in the afternoon.
* Boa noite, Sabrina. (Good evening) - Use this in the evening.
Informal:
* Oi, Sabrina. (This is a more casual greeting, like "Hi")
* E aí, Sabrina? (This is a very informal greeting, similar to "What's up?")
It's best to use the formal greetings unless you know Sabrina well.