The first time you meet a person you can say:
hola (hi, hello)
if you already know the person:
hola, que tal (informal) how's going?
hola, buenos días / buenas tardes / buenas noches good morning / good afternoon / good evening
also:
hola, ¿cómo estás? (informal way) hi, how are you?
buenos días / buenas tardes / buenas noches, ¿cómo está? (formal way) good morning / good afternoon / good evening, how are you?