I can understand that it might feel like you can't be certain of your sexuality until you have an experience with a certain gender/s, and it is valid if you want to wait for this before you choose a label. However, if you feel that bisexual best describes you at this point, then that is also valid, even if you haven't had a romantic experience with a girl. I strongly identify as being under the multisexual umbrella (maybe bi/pan/omni), even though my only relationship has been with a male.
It is also valid to change your label over time, so if you see yourself as bisexual now but in the future find that this doesn't fit for you, it's totally okay to change. I do understand about not wanting to tell people that you were 'wrong' if your choice of label changes, but it doesn't have to feel like that. If you do want to share your sexuality with others, how would you feel about saying something similar to bi being your label for now but that it may change in the future? Once you feel a bit more certain, you could then share your label more confidently/definitively. However, if you do feel ready to share a definitive label now, then go for it, and you can always make adjustments in the future.
I am sorry to hear your current circumstances aren't accepting, and I hate that people are targeted because of their sexuality. I would never want anyone to feel they have to hide their sexuality, but it's also important that you do what is right and safe for you, so I guess you have to weigh up how open you want to be. Whether you want to be fully open with your sexuality or keep it to a limited group of people is up to you, and you should feel supported either way. If you're unsure, you could begin by telling someone that you're close to, and you could then gauge how you feel about sharing it with a wider group of people. This is what I did - it came up in conversation with a trusted friend when I was questioning, and it was then another six months or so before I felt confident in my label and told my best friend, and I then felt ready to share it with other friends/family.
In terms of how to be sure of your sexuality - I don't know if I'm able to give you any ways to be certain, sorry. I guess it's one of those things you just figure out through how you feel, and it can be fluid and change over time.
Good luck with everything, and feel free to keep us updated.