First of all, lesbians dress just like everybody else with all kinds of different styles. Just because a girl dresses in masculine fashions in no way means that she is lesbian. Neither does pushing indicate any particular sexuality. Maybe she’s pushing you to get your attention because she likes you or maybe she’s just aggressive. There is no good and definitive answer. I’m afraid that if you want to know you’ll have to ask her. If she tells you that she is attracted to you then the ball is in your court. If you aren’t attracted to her you can gently let her know that you are not interested. If you are attracted to her then it could be the beginning of a wonderful romance!
As for telling your parents that you are bisexual – seeing as you are not in a relationship with anyone there is no rush. If you have a strong suspicion that they will not take it well, I would recommend that you wait until you feel ready. First of all, you will be more comfortable with your own sexuality and so you will be more confident talking to them about it and, secondly, they will probably take you a little more seriously if you are prepared to this discussion. Whatever you decide, remember that you did not ”choose a lifestyle” rather you are embracing your inherent sexuality and you need to approach your parents with confidence and respect for their beliefs.