It is difficult to talk about your bisexuality to a friend who thinks it isn’t normal for to girls to be in love or to be sexually attracted. However, it is better that she hears it from you than from somebody else. If she knows it this way, she will think you lied to her, you betrayed her. So, it is a good intention you have.
If you want to tell her about your bisexuality, there are two big possibilities. First, you say that you want to talk with her; you tell her, then you explain what bisexuality really means ( that is it normal, it doesn’t mean you will try to kiss her, etc.). Maybe your explication will not be really clear because she will not completely listen to you. The other possibility is to explain what is a bisexual (and a lesbian) before tell her you are one. Telling the truth about all the prejudices she can have and helping her understand that same-gender love is normal before telling her about you will made it easier for you. She will be more comprehensive when the moment of telling her will come. There are surely more than two ways of doing it: you have to chose the one you feel more comfortable with. Try to be calm and ready to answer to her questions. Be comprehensive with her: it is normal if she is surprised and discomfortable at the beginning. Explaining to her it will change nothing to your friendship can be very useful too. Be confident : she is your best friend untill 9 years!
We hope your friend will understand what bisexuality really is and that she will accept you for what you are.