Okay, well, to start with – mixing friendships and sex is never a good idea. Something inevitably goes wrong. So if you really care about your friend and don’t want to jeopardize you relationship with her then I would try to focus this sexual attention elsewhere. If you just want to fuck because you’re attracted to her, that’s not a good enough reason to possibly screw up a relationship with one of your best friends.
You said you’re just starting to come to terms with your sexuality so this is your time to experiment. Don’t do it with your best friend though. Not unless you’re in love with her and you actually want to date her. If you want sex, there are many other girls out there that would be perfect for you explore this new avenue with and not have the ramifications of ruining a friendship.
Right now, you’re not thinking relationships – just your sexual desires – and that’s completely normal for this stage of your life. I don’t think it means that you will NEVER want a relationship with a woman in the future, I think it means that you’re still wading into these waters and your interest is in satisfying the physical side. Besides, you have close friends who are girls. If you get sex with a girl and still have best friends that are girls, you’re essentially meeting all of your needs – just not in one package (yet).
So, my advice, unless you have really strong feelings for your friend and you want a relationship with her, do not mess it up by trying to introduce sex. Especially since she has a boyfriend and hasn’t shown a sexual interest in you.
Look around, see if someone else pops out to you. I guarantee you’ll meet someone else that you’re attracted to with less strings attached than trying to hook up with your best friend. đ
But, if you want to ignore all of this, I always favor the straightforward approach. Ask her if she’s ever thought about girls before – specifically being with one, tell her you’re bi (if she doesn’t know already), and if she responds positively to these things (if she seems into it) then maybe bring it up as a light joke – “well you’re hot, I would sleep with you!” and see how it’s received.
Test the waters in a very light, jesting way and don’t say anything too serious that you can’t get yourself out of by passing it off as a joke if she’s not interested.

