How do you say good girl in Chinese?

Literally "good girl" would be translated into "好女孩" (hao3 nv3 hai2)

However, if you are trying to say "good behaving girl" you can say "乖女孩" (guai1 nv3 hai2)

Also, 女孩 can be substituted in for 女生 depending on the context of the sentence/useage