Just start a conversation with her so you don't seem just like some guy calling her pretty. And next time, you should either say she's pretty on her fb picture so you don't have to say anything afterward, or when you see her in person, try to notice something different about her or if she looks especially good that day. Then be like, "Did you do something different with your hair? It looks good." and things like that to show her you notice those things, even if you really don't. Don't say things like, "You look really pretty TODAY" though. Some girls might not think about it badly, but I do and I know other girls that do too because it sounds like you're saying she looks good that day, but every other days she looks average.