Firstly, you should choose an nice profile picture for yourself, because this will be part of her first impression of you. As for the message, just make it as innocent as possible. Don't start off straight away by flirting too much, because that's what makes it creepy, along with the fact that she doesn't know you. It's pretty hard to avoid the fact that you find her attractive, because you're only judging her by her profile picture, and she'd probably realise this. You should also try to recognise that what you're doing can be a little creepy. This will make you a more understanding guy.
So, just say something like, "Hey, do you ever like to meet guys through Facebook? I'd like to get to know you, but if you think it's too creepy then that's understandable. Thanks!"