I need help with this girl!!?

So I am in high school (10th grade) and just a few weeks ago I started sitting at lunch with my cousin at lunch who isn't real social and he sits by these 2 girls. Found out one has a boyfriend, but the other one is single and I really like her she is a grade ahead of me (11th grade) and I talk to her a little bit and when she talks I listen all the time and make her smile sometimes, but its hard to tell her how I feel with my cousin there and her best friend sitting at the table also (just the 4 of us at the table) I ask her how her day was and we talk, but I want to see if she likes me and i want her number. How can I tell her I like her and get her number? I dont see her besides lunch, but want to see her more like in the hallway and before school to talk to her not just lunch, how do i do that?

I am thinking of asking her bestfriend if she likes me and teller her that I like her? Is this smart?

I really need help because this girl is just the one i have been looking for. I have been single since 8th grade, so 2 years and I been listening to this girl and not really scared to talk to her, but maybe a little, but just i need help on how to ask her for her number and how to talk to her in the hallways or before school because she is always by her best friend