This is always a tough one. First there are some questions I need answered: one, have you come out to her? And two has she come out to you? Is she in to girls?
If you haven't come out to her yet, start there. Take it in steps. The responses you get to each step will help you decide if and how to tell her.
If you come out and it makes her uncomfortable then she'll need time to get used to it. This would also be a pretty good sign that she doesn't want to date.
If you've already come out to her and she's comfortable with it but she hasn't come out or told you definitively that she fancies girls then maybe you should approach this subject before telling her how you feel. Coming out is hard enough without your best friendship hanging on the line.
If you're both out and you think she fancies you, I say ask her out on a date! Something casual that unmistakably says "date", like "hey, do you want to go for dinner and a movie some time?" then after a couple dates it will feel easy and natural to say, "you are so wonderful, I'm crazy about you."
