Who would you feel most comfortable telling first? I'm out to a few friends, but then again like 40% of my friends are bi. XD After that it would probably be my aunt or future sister-in-law. It's weird for me because I'm not completely bi, so it's like "Hey I'm straight.. most of the time, but not always. Ok bye!" I'm pretty sure that's how the conversation would go. XD But with the people I hang around it's assumed that you're at least partly gay to be around them. But yes so who would you come out to first?
I would suggest keeping your mouth shut until you figure yourself out better. Being "not completely bi" pretty much renders you straight or Kinsey 1, which together with your age, marks curiosity. Give it time to develop, says I. Experiment, and when you are at least 80% sure, go ahead and come out.
My lesbian neighbours of course I thought I ought to come out to my housemate first, but I've told a couple people now and he isn't one of them.. its hard! I would agree with above though.. Dont label yourself on feeling - label yourself on experience.. ie when you look back and say 'ive enjoyed being with guys and girls' then call yourself bi.. For now, the label doesnt matter, just follow/explore your desires