Zuhail's and shena's are pretty right. But not so Chinese. In China, we usually say 美女(měinǚ) or 靓女（liàngnǚ）.
This site has a whole list of cute names for a Chinese girlfriend. Have fun!
The Chinese Pinyin for pretty girl is 'piao4 liang4 nv3 hai2' . To know how it looks in Chinese. Click on the picture at left.